The first occurrence.
And YHVH said to her: Two nations are in your womb, and two manner of people shall be separated from your loins; and the one people shall be stronger than the other people; and the elder shall serve the younger. Gen 25:23
The primitive root.
Strong’s H553 אמץ amats, a primitive root meaning, “to be firm.” The 3-letter root is aleph + mem + tsadey.
aleph א = the ox head, thus strength, power, leader
mem מ, ם = the water, thus chaos, mighty, blood
tsadey צ, ץ = the trail, thus a man concealed, journey, chase, hunt
The story: To strengthen (aleph) the mighty (mem) side (tsadey, as a hunter lying in wait on the trail is on his side, not upright). Related words are “to squeeze” and “to suck,” as pressure is applied to both sides at once. Thus to be firm physically is to be sure-footed, as you have planted your body, using the strength of your mighty or muscular sides to remain well-grounded. To be firm mentally is to be courageous, as you have planted your mind to also not be moved.