mere (adj.)

c. 1400, "unmixed, pure," 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," 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"). Original sense of "nothing less than, absolute" (mid-15c., now only in vestiges such as mere folly) existed for centuries alongside opposite sense of "nothing more than" (1580s, as in a mere dream).

mere (n.)

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."