lichen [E17th] This is a borrowing, via Latin lichen from Greek liekhen ‘what eats around itself’ from the way lichen grows. In ancient Greek and in English the word also refered to a skin disease. Lichen has long been used as a dye, and in Old Norse and other Germanic languages lichen was known as ‘dye-moss’, the source of litmus [ME] formed from an old word lit ‘to dye’ and ‘moss’. Figurative use of ‘litmus test’ seems to date from the mid 20th century.