Pronunciation: (fôl'staf, -stäf),[key] —n. 1. Sir John, the jovial, fat knight of brazen assurance and few scruples in Shakespeare's Henry IV, Parts 1 and 2, and The Merry Wives of Windsor. 2. (italics) an opera (1893) by Giuseppe Verdi, with a libretto by Arrigo Boito based on the Shakespearean character.