The first occurrence.
After these things the word of YHVH came to Abram in a vision, saying: Fear not, Abram: I am your shield, and your exceedingly great reward. Gen 15:1
The primitive root.
Strong’s H7939, sakar, a concrete noun meaning, “wages, reward;” from Strong’s H7936 שכר sakar, a primitive root meaning, “to hire.” According to the Ancient Hebrew Lexicon, the shin was originally a sin, transposed over time because of their similar sounds. Thus the 3-letter root is sin + kaph + resh.
sin ס = the thorn, thus grab, hate, protect
kaph כ ,ך = the open palm, thus bend, open, allow, tame
resh ר = the head of man, thus head, first, top, beginning, man
The story: Protective (sin, as thorn bushes were used to protect the flock of sheep out at pasture at night from predators) covering (kaph, as the father’s open palm covers the head of his son when conferring the blessing) over the head of a man (resh). A related word from the sin + kaph pair is booth, sukkot; so that just as a booth is a protective covering over the head of the man from the elements of weather, so a wage, or material compensation, secures a protective covering over the head of the man; that is, it is the substance from which he provides for his house.
Our English words “succor,” and “secure” come from the Hebrew sakar.