The story
From the resonance structure between the covalent form (-yl, where the carbanion exists as a methylidenyl group) and the ionic form (-ide, as in methanide).
Etymology from Wiktionary, CC BY-SA 4.0.
ylide
Today it means: Any organic compound in which two adjacent atoms are connected by both a covalent and an ionic bond; normally written X⁺–Y⁻ Full definition →