late 14c., "act of reserving or keeping back," from Old French reservation (14c.) and directly from Late Latin reservationem (nominative reservatio), noun of action from past-participle stem of Latin reservare "keep back, save up; retain, preserve," from re- "back" (see re-) + servare "to keep, save, preserve, protect" (from PIE root *ser- (1) "to protect").
The mental sense of "qualification of a statement, etc., not expressed openly" is from c. 1600. The U.S. sense of "tract of public land set aside for some special use," especially exclusive use by native peoples, is from 1789, in reference to the Six Nations in New York State. The meaning "act or fact of engaging a room, a seat, etc." is from 1904, also originally American English.