late 14c., of a voice, "pure, clear;" mid-15c., of abstract things, "absolute, sheer;" from Old French mier "pure" (of gold), "entire, total, complete," and directly from Latin merus "unmixed" (of wine), "pure; bare, naked;" figuratively "true, real, genuine," according to some sources probably originally "clear, bright," from PIE *mer- "to gleam, glimmer, sparkle" (source also of Old English amerian "to purify," Old Irish emer "not clear," Sanskrit maricih "ray, beam," Greek marmarein "to gleam, glimmer"). But de Vaan writes "there is no compelling reason to derive 'pure' from 'shining,'" and compares Hittite marri "just so, gratuitously," and suggests the source is a PIE *merH-o- "remaining, pure."
The English sense of "nothing less than, in the fullest sense absolute" (mid-15c., surviving now only in vestiges such as mere folly) existed for centuries alongside the apparently opposite sense of "nothing more than" (1580s, as in a mere dream).
"pool, small lake, pond," from Old English mere "sea, ocean; lake, pool, pond, cistern," from Proto-Germanic *mari (source also of Old Norse marr, Old Saxon meri "sea," Middle Dutch maer, Dutch meer "lake, sea, pool," Old High German mari, German Meer "sea," Gothic marei "sea," mari-saiws "lake"), from PIE root *mori- "body of water." The larger sense of "sea, arm of the sea" has been obsolete since Middle English. Century Dictionary reports it "Not used in the U.S. except artificially in some local names, in imitation of British names."
"boundary line" (between kingdoms, estates, fields, etc.), now surviving in provincial use or place names, but once an important word, from Old English mære "boundary, object indicating a boundary," from Proto-Germanic *mairjo- (source also of Middle Dutch mere "boundary mark, stake," Old Norse -mæri "boundary, border-land"), related to Latin murus "wall" (see mural (n.)).
Hence merestone "stone serving as a landmark" (Old English mærstan); mere-stake "pole or tree standing as a mark or boundary" (1620s); meresman "man appointed to find boundaries" (of a parish, etc.). In Middle English meres of erthe (c. 1400) was "the ends of the earth."