1610s, "sum, amount," from Latin quantum (plural quanta) "as much as, so much as; how much? how far? how great an extent?" neuter singular of correlative pronominal adjective quantus "as much" (see quantity).
The word was introduced in physics directly from Latin by Max Planck, 1900, on the notion of "minimum amount of a quantity which can exist;" reinforced by Einstein, 1905. Quantum theory is from 1912; quantum mechanics, 1922. The term quantum jump "abrupt transition from one stationary state to another" is recorded by 1954; quantum leap "sudden large advance" (1963), is often figurative.
mid-15c. (implied in tauntingly), possibly [Skeat] from French tanter, tenter "to tempt, try, provoke," variant of tempter "to try" (see tempt). Or from French tant pour tant "so much for so much, tit for tat," on notion of "sarcastic rejoinder" (considered by OED the "most likely suggestion"), thus from Old French tant "as much," from Latin tantus, from tam "so;" see tandem. Related: Taunted; taunting.