Entry 1 sense Webster, 1913 Modus operandi /mōd'-əs äpˌ-ẽr-ăn'-dē/ · Mo·dus op·er·an·di · IPA /ˈmoʊ.dəs ɑp.əɹ.ˌæn.daɪ/ 01 manner of operating. Often abbreviated to MO 1. manner of operating. Often abbreviated to MO