Ent

wiktionary

ref

Learned borrowing from Old English ent(“giant”), from Proto-West Germanic *anti; introduced by J. R. R. Tolkien in The Lord of the Rings, 1954–55. 

Compare Middle English *ent, eont(“giant”), inherited from the Old English word, but which apparently didn't survive through the Middle English period into Modern times. Apparently survived in some German dialects as Enz(“giant”), also in composite forms. Compare ettin.

Possibly from empty, through assimilation of the "m" to the following "t"