type of citrus fruit, c.1400, from O.Fr. limon "citrus fruit," from O.Prov., from Arabic laimun or Pers. limu(n), generic terms for citrus fruits (compare lime); cognate with Skt. nimbu "the lime." Slang meaning "a Quaalude" is 1960s, from Lemmon, name of a pharmaceutical company that once manufactured the drug.
"worthless thing," 1909, Amer.Eng. slang; from lemon (1), perhaps via criminal slang sense of "a person who is a loser, a simpleton," which is perhaps from the notion of someone a sharper can "suck the juice out of." A pool hall hustle was called a lemon game (1908); while to hand someone a lemon was British slang (1906) for "to pass off a sub-standard article as a good one." Or it simply may be a metaphor for something which "leaves a bad taste in one's mouth."