From Latin 'defungi' (to finish performing) — the dead sibling of 'function,' both from Latin 'fungi' (to perform).
No longer existing or functioning.
From Latin 'defunctus,' past participle of 'defungi' (to finish, be done with, die), from 'de-' (completely) + 'fungi' (to perform, discharge). Something defunct has 'finished performing' — it's done its function and is over. Key roots: de- (Latin: "completely, off"), fungi (Latin: "to perform, carry out").
'Defunct' and 'function' are the same word — one alive, one dead. Latin 'fungi' (to perform) gives us 'function' (something performing) and 'defunct' (something that has finished performing). 'Perfunctory' (done without care) literally means 'gotten through' — performing just enough to be done with it. Even 'fungible' (interchangeable) comes from the same root. It's a family of words about doing your job — or being done with it.