radio-active UK /ˌɹeɪdiəʊˈaktɪv/US /ˌɹeɪdioʊˈæktɪv/ 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. formsmore radio-active · most radio-active