Oxygen
late 18th century: from French ( principe) oxygène ‘acidifying constituent’ (because at first it was held to be the essential component in the formation of acids).
wiktionary
Borrowed from French oxygène (originally in the form principe oxygène, a variant of principe oxigine ‘acidifying principle’, suggested by Lavoisier), from Ancient Greek ὀξύς(oxús, “sharp”) + γένος(génos, “birth”), referring to oxygen's supposed role in the formation of acids.
etymonline
oxygen (n.)
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 -genes "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.
