Charles Ellicott Commentary Genesis 49:4

"Boiling over as water, thou shalt not have the pre-eminence; Because thou wentest up to thy father`s bed; Then defiledst thou it: he went up to my couch." — Genesis 49:4 (ASV)

Unstable. —This translation is shown to be correct by the use of the word in Judges 9:4; Zephaniah 3:4, in both of which places it is translated light. From this sense of lightness and frivolity, the meaning of wantonness, which the word has in Syriac, naturally arose. In Arabic, it means boastful, another aspect of feebleness.

This sense agrees well with the comparison to water, for it is water's nature to seek its own level; while yielding to every impression, it retains none. The other meaning attributed to it by many able critics is “boiling over like water,” a description of the unrestrained violence of Reuben’s passions.

Thou shalt not excel. —That is, you shall not have that excellence which was yours by right of birth.