ajar (adv.)

"slightly open, neither open nor shut," 1718, also on a jar, on the jar, perhaps from Scottish dialectal a char "turned a little way," earlier on char (mid-15c.) "on the turn (of a door or gate)," from Middle English char "a turn," from Old English cier "a turn" (see chore). For first element see a- (1). For unusual change of ch- to j-, compare jowl.