1610s, "equal to what is needed or desired, sufficient," from Latin adaequatus "equalized," past participle of adaequare "to make equal to, to level with," from ad "to" (see ad-) + aequare "make level," from aequus "equal, even" (see equal (adj.)). The sense is of being "equal to what is required." It shares duty with enough, depending on the subject. With a slightly disparaging tinge, "mediocre, just good enough," by 1900. Related: Adequateness.