The first occurrence
And a river went out of Eden to water the garden; and from there it was parted, and became four heads. Gen 2:10
The primitive root
Strong’s H7218 ראש rosh, a concrete noun meaning, “head;” from an unused, unnamed primitive root meaning, “to shake.” The 3-letter root of the noun is resh + aleph + shin.
resh ר = the head of man, thus head, first, top, beginning, man
aleph א = the ox head, thus strength, power, leader
shin ש = two front teeth, thus sharp, press, eat, two, again
The story: The head (i.e. chief man, resh) has the power (aleph) to give his assent or denial (shin, i.e., a head nod of assent or shake of denial is a movement made again and again).