It is the hypothetical source of/evidence for its existence is provided by: Sanskrit ledhi "he licks," Armenian lizum "I lick," Greek leikhein "to lick," Latin lingere "to lick," Old Irish ligim "I lick," Welsh llwy "spoon," Old English liccian "to lick."
The dye is obtained from certain lichens. It is naturally blue but turns red in acid and is restored to blue by alkalis. Figurative use of litmus test is first attested 1957, from scientific use of litmus-treated paper as a chemical indicator. Litmus paper with this meaning is from 1803.
algae genus, 1716, from Latin fucus, a type of reddish seaweed or rock-lichen, from or related to Greek phykos "seaweed," which is said to be of Semitic origin (see phyco-). From it was prepared in ancient times a red dye for woolen goods; hence in Greek and Latin it also had a sense "red paint" and was the general word for the article of women's make-up that supplied the place of rouge; and in Latin it was further extended to include "deceit, disguise." The custom is said to have originated among the Ionians, who were in close contact with Semitic peoples.
The word was in Middle English as fuke, fuike "dissimulation" (mid-15c.); "red woolen cloth" (late 15c.); later fucus "a paint, dye," especially for the face, "rouge," also commonly used 17c. figuratively as "disguise, pretense." Hence also obsolete fucate "disguised, dissembling" (1530s), literally "colored, beautified with paint," from Latin fucatus "painted, painted, colored, disguised," past-participle adjective from fucare, a verb derived from fucus.