late 15c., squyrten "to spit" (intransitive), of uncertain origin, probably imitative. Transitive sense "cause to issue in a sudden jet or stream" is from 1580s. Related: Squirted; squirting. Squirt-gun attested from 1803.
late 14c., "diarrhea," from squirt (v.). Meaning "jet of liquid" is from 1620s. Meaning "a whipper-snapper" is from 1839.