in mechanics, "a prop, a support" (on which a lever turns), 1670s, from Latin fulcrum "bedpost, foot of a couch," from fulcire "to prop up, support" (see balk (n.)).
also baulk, Middle English balke, from Old English balca "ridge, bank," from or influenced by Old Norse balkr "ridge of land," especially between two plowed furrows, both from Proto-Germanic *balkon- (source also of Old Saxon balko, Danish bjelke, Old Frisian balka, Old High German balcho, German Balken "beam, rafter"), from PIE root *bhelg- "beam, plank" (source also of Latin fulcire "to prop up, support," fulcrum "bedpost;" Lithuanian balžiena "cross-bar;" and possibly Greek phalanx "trunk, log, line of battle"). Italian balco "a beam" is from Germanic (see balcony).
In old use especially "an unplowed strip in a field, often along and marking a boundary." The modern senses are figurative, representing the balk as a hindrance or obstruction (see balk (v.)), or else the notion of "a piece missed in plowing" as "a blunder, a failure." Hence, in baseball, "a motion made by the pitcher as if to deliver the ball, but without doing so," attested from 1845, probably from the plowing sense.