Unfit means not healthy enough, or not good enough for a purpose. Out of shape, or not up to the job.
The fitness sense is about the body, and being unfit is the plain British word for it. The other sense is a formal judgement and a serious one, since unfit for work, unfit to plead and an unfit parent are all decided by doctors or courts. There is a fixed phrase for buildings, since a house can be declared unfit for human habitation. It takes for or an infinitive after it.
Middle English
fit — an adversary of equal power
English
fit
English, today
unfit
From un- + fit. fit Possibly from Middle English fit (“an adversary of equal power”), of unknown further origin. The original sense appears to have been "to marshal or deploy troops", which shifted to "to suit (troops to a certain location" > "suitable", and may be further related to Icelandic fitja (“to web, knit, cast on”).
Etymology from Wiktionary, CC BY-SA