"way of doing or accomplishing," 1650s, Latin, literally "mode of operating" (see modus). Abbreviation m.o. is attested from 1955.
Entries linking to modus operandi
modus n.
"way in which anything is done," 1640s, from Latin modus (plural modi) "measure, extent, quantity; proper measure, rhythm, song; a way, manner, fashion, style," from PIE root *med- "take appropriate measures." Especially in modus operandi and modus vivendi.