Quiz: Remember the definition of mal de mer?
mid-15c., "to escheat," a shortening of Old French escheat, legal term for revision of property to the state when the owner dies without heirs, literally "that which falls to one," past participle of escheoir "befall by chance, happen, devolve," from Vulgar Latin *excadere "to fall away," from Latin ex- "out" (see ex-) + cadere "to fall" (see case (n.1)). Also cf. escheat. The royal officers evidently had a low reputation. Meaning evolved through "confiscate" (mid-15c.) to "deprive unfairly" (1580s). To cheat on (someone) "be sexually unfaithful" first recorded 1934. Related: Cheated; cheating.
late 14c., "forfeited property," from cheat (v.). Meaning "a deceptive act" is from 1640s; earlier, in thieves' jargon, it meant "a stolen thing" (late 16c.), and earlier still "dice" (1530s). Meaning "a swindler" is from 1660s.
To be sexually unfaithful; get a little on the side (1930s+)