radioactive UK /ˌɹeɪdiəʊˈaktɪv/US /ˌɹeɪdioʊˈæktɪv/adjective 2noun 1forms 3 adjective2 •Exhibiting radioactivity.As a matter of fact many of our best drinking waters have all sorts of unspecified qualities. Burton water, for example, is radioactive by Beetham's standards up to the ninth degree. — 1917 •(figurative) Dangerous and disgusting, particularly of people or ideas.Even sleazy tactics and his radioactive mouth may not be able to contain this debacle. noun1 •Any radioactive substance.Any ship nearby will receive a lethal dose of gamma rays, neutrons, and other radioactives. — 2016 formsradioactives · more radioactive · most radioactive