chemical formed in the pineal gland of mammals that regulates certain physiological activities, 1958, from Greek melas "black, dark" (see melano-) + ending from serotonin. So called because its secretion is inhibited by sunlight, or because it changes the skin color of certain reptiles and amphibians.
Old English creopera "one who creeps," creopere "a cripple," agent nouns from creep (v.). Also see creep (n.). Meaning "lice" is from 1570s; of certain birds which scramble in trees and bushes from 1660s; of certain plants which grow upon the surface, sending out rootlets from the stem, from 1620s.
"put to work at certain duties, compel to work for one's benefit," 1806, from British public school slang fag (n.) "junior student who does certain duties for a senior" (1785), from fag (v.1). Related: Fagdom (1902); faggery "fatiguing labor" (1853). Brain-fag (1850) was an old term for "mental fatigue."