Where does “ajar” come from?

Ajar comes from English a combined with English jar, with a traced to Latin ad- and ad, ultimately from Proto-Indo-European h₂epo and al-.

ajar (English): Slightly turned or opened; To turn or open...

Definitions

Step	Language	Word	Meaning
1	Middle English	ajar	—
2	Middle English	char	turn, occasion
3	Old English	ċierr	a turn, change, time, occasion, affair, business
4	Proto-Germanic	karzijaną	to turn, sweep
5	Proto-Indo-European	ǵers-	to turn, bend, twist; to twist; to plait things...

ajar — Etymology, Origin & Word History | Etymology Explorer