Charles Ellicott Commentary Genesis 49:3

"Reuben, thou art my first-born, my might, and the beginning of my strength; The pre-eminence of dignity, and the pre-eminence of power." — Genesis 49:3 (ASV)

The beginning of my strength. —In Genesis 35:18, the word oni means “my sorrow,” and it is so translated here by the Vulgate, Aquila, and Symmachus. But in this verse Jacob magnifies the prerogatives of the firstborn, and our version is undoubtedly right in deriving oni from a different and not uncommon word signifying strength. It occurs in Deuteronomy 21:17; Job 40:16; Psalms 78:51; Psalms 105:36, etc.

The excellency ... — We must here supply, “And therefore to you as the firstborn belonged,” first, the excellency of dignity, that is, the priesthood; and secondly, the excellency of power, that is, the kingly office. As a matter of history, no king, judge, or prophet is recorded as having sprung from the tribe of Reuben.