abil

The root abil is, in truth, the heart of English's most productive adjective suffix: -able / -ible. It comes from Latin habilis, meaning 'handy, easy to hold, manageable' — itself from habēre, 'to have, to hold.' Something habilis was something you could get a grip on and handle; from 'easy to handle' it slid to 'capable, fit, able.'

The core word is able itself: if you are able to do something, you can 'handle' it. Strip nothing away and you have the bare root standing as an everyday word. Add the negative un- and you get unable ('not able'); add the causative en- ('to make') and you get enable ('to make able'). Turn it into a noun with -ity and you get ability — the state of being able, the capacity to handle a task.

Here is the key idea for a learner: abil mostly works as a suffix, attaching to other words to mean 'able to be ___ed.' Take any verb, add -able, and you get an adjective of possibility — and add -ability to make the matching noun. That is why so many words in this family carry a second root that does the heavy lifting of meaning, while abil only adds 'able to be':

- prob- (prove/likely) + ability → probability (how able-to-happen something is)
- cap- (take, hold) + ability → capability (able to take on a task)
- cul(p)- (blame) + able → culpable (able to be blamed)
- bear + able, with un- → unbearable (not able to be borne)
- believe + able, with un- → unbelievable (not able to be believed)
- avoid + able, with un- → unavoidable (not able to be avoided)
- malle- (hammer) + ability → malleability (able to be hammered into shape)
- permea- (pass through) + ability, with im- → impermeability (not able to be passed through)

Notice the spelling choice: Latin verbs of the first conjugation usually take -able (avoidable, bearable), while many Latin-derived stems take -ible (visible, possible). Both are the same habilis root wearing two coats.

So whenever you see -able / -ible / -ability, read it as 'able to be': the word in front tells you what, and abil tells you it can happen.