late 14c., of persons, "having the left hand stronger or more capable than the right;" 1650s of tools, etc., "designed for use with the left hand," from left (adj.) + -handed. In 15c. it also could mean "maimed." Sense of "underhanded" is from early 17c., as in left-handed compliment (1787, also attested 1855 in pugilism slang for "a punch with the left fist"), as is that of "illicit" (as in left-handed marriage, for which see morganatic; 17c. slang left-handed wife "concubine"). Related: Left-handedly; left-handedness.