late 12c., from a Scandinavian source akin to Old Norse þrysta "to thrust, force, press," from Proto-Germanic *thrustijanan, perhaps from PIE *treud- "push, press" (see threat), but OED finds this derivation doubtful. Related: Thrusting.
1510s, "act of pressing," from thrust (v.). Meaning "act of thrusting" (in the modern sense) is from 1580s. Meaning "propulsive force" is from 1708. Figurative sense of "principal theme, aim, point, purpose" is recorded from 1968.