Wiktionary(0.00 / 0 votes)Rate this definition: 1  mutant(Noun) Something which has mutated, which has one or more new characteristics from a mutation. 2  mutant(Noun) Someone or something that seems strange, abnormal, or bizarre. 3  mutant(Adjective) That has undergone mutation.  mutant(Adjective) Strange, abnormal, or bizarre.