Meaning

noun any of several eruptive skin diseases characterized by hard thick lesions grouped together and resembling lichens growing on rocks

noun any thallophytic plant of the division Lichenes; occur as crusty patches or bushy growths on tree trunks or rocks or bare ground etc.

originBorrowed from Latin līchēn, from Ancient Greek λειχήν (leikhḗn), from λείχω (leíkhō, “to lick”). Originally used of liverwort; the modern sense first recorded 1715.

Etymology from Wiktionary, CC BY-SA 3.0