c.1300, "branched or divided in two parts," past participle adjective from fork, v.. Of roads from 1520s; from 1550s as "pointing more than one way." In 16c.-17c. sometimes with a suggestion of "cuckold," on the notion of "horned." Forked tongue as a figure of duplicitous speech is from 1885, American English. Double tongue in the same sense is from 15c.