codicil /ˈkɒdɪsɪl/noun 3verb 1forms 5 noun3 •An addition or supplement that explains, modifies, or revokes a will or part of one. •An addition or supplement modifying any official document, such as a treaty.So insistent was this demand that the Wyandot actually received a codicil to the treaty […] — 2004 •Any appendix or addition.If Nick answered a question Wani listened to him and then gave a flat little codicil or correction. — 2004 verb1 •To add a codicil (to). formscodiciled · codicilled · codiciling · codicilling · codicils