The Hebrew noun on (אוֹן) denotes strength, generative vigor, or productive power. It is used particularly of the firstborn as the first fruits of one's strength (Genesis 49:3), representing the peak of a father's generative power and resources.
Human strength and wealth expressed by on are gifts of God meant to be consecrated to Him. Reuben forfeited his right as firstborn through moral failure (Genesis 49:3–4). Proverbs warns against wasting one's on on immoral living (Proverbs 31:3). True strength is strength consecrated to God, not squandered on the ephemeral.