[droithomme]

Well, ept is obviously a back-formation and a clever and amusing one.

Etymology is straight from Latin: ineptus, which is prefix in- plus aptus (fitting or suitable). Interestingly there's also inapt which is quite similar.

edit: aheilbut's research on this is much more thorough.

[MartinCron]

I used a similar back figuring when describing a co-worker who was in the wrong job... "He's not inept, he is inapt"