-gen

gaseous chemical element, 1790, from French oxygène, coined in 1777 by French chemist Antoine-Laurent Lavoisier (1743-1794), from Greek oxys "sharp, acid" (from PIE root *ak- "be sharp, rise (out) to a point, pierce") + French -gène "something that produces" (from Greek -genēs "formation, creation;" see -gen).

Intended to mean "acidifying (principle)," it was a Greeking of French principe acidifiant. So called because oxygen was then considered essential in the formation of acids (it is now known not to be). The element was isolated by Priestley (1774), who, using the old model of chemistry, called it dephlogisticated air. The downfall of the phlogiston theory required a new name, which Lavoisier provided. Oxygen-mask is attested from 1912.

1888 in geology, "transported to its present position from elsewhere," from Greek allogenēs "of another race, stranger," from allos "other, another, different" (see allo-) + -genēs "born" (see -gen) + -ic. An earlier adjective was allogeneous "of a different kind or nature" (1842).