Unstable. — This translation is shown to be right by the use of the word in Judges 9:4; Zephaniah 3:4, in both which places it is translated light. Out of this sense of lightness and frivolity naturally arose the meaning which the word has in Syriac of wantonness. In Arabic it means boastful, another side of feebleness. With this sense the comparison with water well agrees; for it is its nature to seek a dull level, and while yielding to every impression to retain none. The other meaning given 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, thou shalt not have that excellency which was thine by right of birth.