/ Dictionary / Index M methicillin: Meaning and Definition of Find definitions for: meth•i•cil•lin Pronunciation: (meth"u-sil'in), [key] — n. Pharm. a semisynthetic penicillin antibiotic, CHNNaOS, used principally in the treatment of severe, penicillin-resistant staphylococci infections. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: methicillin (Thesaurus) methenamine methimazole Related Content Daily Word Quiz: morose Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language