before 900;Middle Englishfangen to seize, catch; cognate with Old Saxonfangan,Germanfangen, variant of proto-Germanic*fanhan-, whence Old Englishfōn, cognate with Old Saxon,Old High German,Gothicfāhan,Old Norsefā; akin to Old Englishgefangian to fasten
Old English fang "prey, spoils, plunder, booty; a seizing or taking," from gefangen, past participle of fon "seize, take, capture," from Proto-Germanic *fango- (cf. Old Frisian fangia, Middle Dutch and Dutch vangen, Old Norse fanga, German fangen, Gothic fahan), from PIE root *pag- "to make firm, fix;" connected to Latin pax (genitive pacis) "peace" (see pact).
The sense of "canine tooth" (1550s) probably developed from Old English fengtoð, literally "catching- or grasping-tooth." Transferred to the venom tooth of a serpent, etc., by 1800.
A long, pointed tooth in vertebrate animals or a similar structure in spiders, used to seize prey and sometimes to inject venom. The fangs of a poisonous snake, for example, have a hollow groove through which venom flows.