Thanks for the explanation, it helped, but I still have no clue where does the "ajar" thing come from, letter for letter.

Ajar, to mean, slightly open. So, not a open door (anyone), not a revolving door (everyone), and not a closed door (no one).

It's a bit of a contrived term and there's not much to read into it. It's like how "hitting a home run" is slang for getting laid, and rounding the bases was an associated metaphor that evolved from it; they only make sense by the association with the original metaphor.

...Oh, it's an actual English word, I didn't know and now I am embarrassed. Should have checked the dictionary.

Thank you!