Genèse 49:3

וראשית אוני THE BEGINNING OF MY VIGOR — That is the first drop [of semen], as he had not experienced a seminal emission from all of his days [until that time] (Genesis Rabbah 98:4).

אוני means MY VIGOR, as (Hosea 12:9) “I have gained me (און) strength”; (Isaiah 40:26) “by the greatness of His (אונים) might”; (Isaiah 5:29) “and to him that hath no (אונים) might Ho increaseth strength”.

יתר שאת SUPERIORITY IN DIGNITY — Since you were the first-born it was proper that you should be superior to your brothers by being endowed with the priesthood. This term שאת “lifting up” alludes to the נשיאות כפים the raising of the hands when the priests pronounce the benediction (cf. Onkelos).

ויתר עז SUPERIORITY IN POWER — meaning in royal rank, as is meant by עז in (1 Samuel 2:10) “And he will give strength (עז) unto His king”. And what caused you to lose all these?

Reuben thou art my firstborn, the beginning of the strength of my generation, and the chief event of my thoughts To thee belonged the birthright, and the high priesthood, and the kingdom: but because thou hast sinned, my son, the birthright is given to Joseph, and the, kingdom to Jehuda, and the priesthood to Levi.

REUBEN MY FIRSTBORN THOU ART.. Ramban will transpose the order of the words for the purpose of deriving the meaning of the verse. The sequence of this verse is, “Reuben, thou art my firstborn, my might, and the first-fruits of my strength, the excellence of majesty, and the excellence of power,” and its purport is as follows: “You are the first-born of my might and the first-fruits of my strength when I was the excellence of majesty (se’eith) and position” — the expression being similar to that found in the verses: Shall not his excellency (‘se’eitho’) make you afraid? When he raiseth himself up (‘mise’eitho’) the mighty are afraid,, 41:17. ‘mise’eitho’ meaning ascendancy and power — “and when I was at the height of strength (oz) for war” — the expression being similar to that found in the verses: And He will give strength (‘oz’) unto His king; And the strength (‘ve’izuz’) of battle.

כחי וראשית אוני יתר שאת ויתר עז, although it sounds as if Yaakov is describing more than one facet of Reuven, it is a single statement incorporating what writers call “poetic license.” כחי, my power, my economic might; We find the word און as also referring to money, i.e. economic power in Hoseah,12,9 אך עשרתי, מצאתי און לי, ”I have become rich, I have gotten power.” Also in Job 20,10 the word און appears in the sense of economic power, וידיו תשבנה אונו, “his own hands must give back his wealth.”

What Yaakov is referring to is that although Reuven as the first-born was entitled to a double portion of his father’s estate, his conduct resulted in his forfeiting this advantage and forfeiting his chance of being the leader among his brothers. פחז כמים, having been unstable like water, you will no longer enjoy your excellence, i.e. יתר שאת, neither the rights of the first-born nor the claim to be king over the tribes.

יתר שאת ויתר עז, פחז כמים אל תותר; you are as unstable as water; you will therefore not enjoy an advantage over your brothers, the advantage which would have consisted of יתר שאת, enhanced social position and the entitlement to the firstborn’s extra share in the inheritance as well as the status of priesthood. Even though you are biologically, ראשית אוני, and as such would have been entitled to privileges involving an extra share in my inheritance, (based on Deuteronomy 21,17) as well as

יתר עז enhanced political clout by supplying the kings of the Jewish nation from your tribe, you have forfeited all this by having displayed character weakness. (compare Chronicles II 21,3 where the claim to the throne by Yehoram was based on his being the firstborn).

ראובן בכורי אתה; Yaakov repeated this theme a second and a third time when he continued: כחי, וראשית אוני. The reason was that theoretically, Reuven was entitled to three advantages. 1) birthright; 2) priesthood, as every firstborn in each family was automatically what the כהן would be after the sin of the golden calf when that privilege was transferred to the tribe of Levite who had not participated in that sin. 3) the position of political head of the nation, known as מלכות in the language of our sages.

יתר שאת, this expression encompasses both the birthright and the priesthood, both of which are primarily a description of an aristocratic social standing.
ויתר עז, a reference to the political power, Royalty. Kings need to excel in this fortitude, toughness described by the word עוז. We find the expression used in that sense in Samuel I 2,10 ויתן עוז למלכו, “He will give power to His king.” Yaakov is reminding Reuven: “you would have enjoyed all these advantages over your brothers if you had not become guilty of a great sin.” This is why he continued with פחז כמים, you were precipitous as water in not restraining your biological urges which led you to mount the couch of your father. You did not stop to consider what a sin with far ranging effects you would make yourself guilty of by your lack of self control.

אל תותר, you will no longer enjoy any advantage over your brothers.

ראובן בכורי אתה, Reuven you, who are my firstborn,” and who should have received a double share of my inheritance; and you who should also have shouldered the yoke of being the priest among your siblings, and from whom eventually the first King of the Israelites should have been crowned, (in accordance with Chronicles II 21,3) “and the title of Royalty he gave to Yehoram for he was the firstborn,” you will not enjoy all this as it has been transferred to Joseph, and the priesthood to Levi, and the Royalty to Yehudah, all because you have been as unstable as water, as a kettle that boils over and spills its contents.” (Compare Chronicles I 5,1) As a result, the only privilege a firstborn enjoys that you still possess is the hollow title of having been the firstborn of my sons. In this respect you are comparable to water, the first physical element in the universe. You are the original, (oldest) of the people that make up the Israelite nation, the 12 tribes.” Water is insubstantial; it cannot be poured out completely, as opposed to something dry which can be emptied without the loss of any part of it. In the Talmud, tractate Shabbat folio 55, where the meaning of the word פחז in our verse is discussed by different scholars, [there is a lengthy debate about what constituted the sin of a number of illustrious people such as Reuven, the sons of High Priest Eli, Solomon, Yoash, Pinchos, David, King Yoshia, and others Ed.] each offering a different opinion; the conclusion arrived at is that the scholars of the Mishnah agreed to disagree. The common denominator of their discussion is that the text describing their respective sins cannot be understood at its face value but is a metaphor describing a character weakness on the occasion. (B’reshit Rabbah 99,6)