bil

The root bil is the reduced, in-word form of the same Latin source as abil: habilis, 'able, manageable,' from habēre 'to hold.' Where abil- stands at the front of a word (able, ability, capable), bil- surfaces in the middle, almost always inside the noun ending -bility (from Latin -bilitās). So bil is less a root with its own meaning than the spelling that 'able' takes when buried inside a longer abstract noun.

The clearest pattern is this: take an adjective ending in -ble and turn it into a quality-noun, and the -ble becomes -bility:

- stable → stability (the quality of standing firm)
- probable → probability (the quality of being likely)
- accessible → accessibility (the quality of being reachable)
- flexible → flexibility, visible → visibility, possible → possibility

In every one of these, the heavy lifting of meaning is done by another root — st- ('stand') in stability, ced- ('go') in accessibility — while -bil- only carries the faded sense of 'able to be.' That is why, when you see a word like stabilize, stabilization, or destabilize, you should locate the real root (st, 'stand') first; the -bil- is part of the -able/-ability machinery, not a separate idea.

There is one member where bil truly carries the 'able' meaning on its own stem: debilitate. It comes from Latin dēbilis ('weak'), literally de- ('away, un-') + -bilis ('able') = 'un-able, lacking strength.' To debilitate is to drain someone's ability to function — to make them dēbilis, weak. This is the one word in the family where you can feel bil meaning 'able' inside the stem itself.

Beware a false friend: bilious has nothing to do with this root. It comes from Latin bilis, meaning 'bile' (the bitter digestive fluid), and describes someone ill-tempered or sickly — the old idea that excess bile made you irritable. Same spelling, completely separate origin.