Genesis 49:3 - Matthew Poole's English Annotations on the Holy Bible

The beginning of my strength; the first instance or evidence of my might or strength, or of that masculine rigour whereby God enabled me to beget a child. Compare Deuteronomy 21:17 Psalms 105:36. Or the first of my children, which are the strength, the stays, and supports of a father, and of his family; thence called his arrows, as Psalms 127:4, and by other authors, the pillars of the house. The excellency of dignity, and the excellency of power. As first-born thou hadst the right of precedency before all thy brethren in point of dignity and power or privilege; the double portion, the priesthood, the dominion over thy brethren were thine.