Radioactive

wiktionary

ref

Coined by Pierre Curie and Marie Curie in 1898 as French radio-actif, equivalent to English radio- +‎ active.

etymonline

ref

radioactive (adj.)

1898, of an atomic nucleus, "capable of spontaneous nuclear decay releasing ionizing emissions," from French radio-actif, coined by Pierre and Marie Curie from radio-, combining form of Latin radius "ray" (see radius) + actif "active" (see active). Of processes, etc., "involving or produced by radioactivity," by 1903.