early 12c., from Old English gemot "meeting, formal assembly" (especially of freemen, to discuss community affairs or mete justice), "society, assembly, council," from Proto-Germanic *ga-motan (compare Old Low Frankish muot "encounter," Middle Dutch moet, Middle High German muoz), from collective prefix *ga- + *motan (see meet (v.)). In early 15c. awful moot was used for "the Last Judgment."
"debatable, subject to discussion," by 1650s, from moot case (1570s), earlier simply moot (n.) in the specialized sense "discussion of a hypothetical law case" (1530s) in law student jargon. The reference is to students gathering to test their skills in mock cases.
"to debate, argue for and against" (mid-14c.), from Old English motian "to meet, talk, discuss, argue, plead," from mot "meeting" (see moot (n.)). Meaning "raise or bring forward for discussion" is from 1680s. Related: Mooted; mooting.