From the verb wehren.
Wehr f (genitive Wehr, plural Wehren)
- (idiomatic) sich zur Wehr setzen (offer resistance, put up a fight)
- (obsolescent) guard, body of men
- short for: Feuerwehr
According to Pfeifer: Originally from the coast in northern Germany with the meaning “any obstruction in water built with posts etc.”, from Middle High German wer from Old High German wer, both related to the precursor of wehren and cognate with Old English wer, English weir, Dutch weer.
According to Duden: Either identical to die Wehr (in which case the original meaning was “fortification against water”) or in the sense of “wickerwork”, as in the etymology of wehren (in which case the original meaning was “fishing weir”).