Rashi on Genesis 49:3/רש"י על בראשית מ״ט:ג׳

1וראשית 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).

2אוני 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”.

3יתר 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).

4ויתר 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?