The adjective ajar describes something that is slightly open. A door that has been left ajar is easily pushed open by the wind or a nosy person.
To correctly pronounce ajar, say "uh-JAR." People in a hurry leave drawers and cabinet doors ajar, or hanging open. If a chime starts ringing when you start a car, a door or the trunk may be ajar. Check that everything is securely closed before taking off. But don't describe a mouth that is open in surprise as ajar. A better word choice is agape.