Meaning

noun a coloring material (obtained from lichens) that turns red in acid solutions and blue in alkaline solutions; used as a very rough acid-base indicator

originFrom Middle English litmose, lytmose, litemose, from Old Norse litmosi (“moss used for dyeing”), from lita (“to dye, stain”) + mosi (“moss”), the former from litr (“colour, dye, blee”), from Proto-Germanic *wlitiz, *wlituz (“appearance, blee”), from Proto-Indo-European *wel- (“to see”). Cognate with Old English wlite (“appearance, form, brightness, countenance”). More at moss.

Etymology from Wiktionary, CC BY-SA 3.0