The Hebrew verb atsam (עָצַם) means to be mighty, to be strong, or to be numerous. The related noun etsem (H6106) means bone or substance. Together they form a word family about essential, bone-deep strength.
The verb appears in Genesis 26:16 when Abimelech tells Isaac, "thou art much mightier than we," fulfilling the Abrahamic promise. In Exodus 1:7, Israel becomes "exceedingly mighty" in Egypt, fulfilling Genesis 17. The same root underlies Psalm 105:24: "He made them stronger than their enemies." God's power to multiply and strengthen His people is the ground of covenant confidence.