Modus Operandi
M.O.
Someone's habits of working, particularly in the context of business or criminal investigations.
Origin
The Latin phrase — literally "mode of operating" — has been used in English since at least the 17th century. Its formalization as a criminal investigation system is credited to Llewellyn Atcherley, Chief Constable of the West Riding of Yorkshire, who around 1913 developed a method for recording ten key data points about criminals' methods to identify repeat offenders across jurisdictions. August Vollmer, Berkeley, California's pioneering police chief, adapted Atcherley's system for American law enforcement by 1919, and Scotland Yard's Modus Operandi Bureau further spread the approach internationally in the 1920s.
Everyday Use
"That's her M.O." — when you notice someone always handles situations the same way, you're identifying their modus operandi. It shows up in crime dramas constantly, where detectives link cases by a criminal's distinctive methods, but we use it just as often at work: "His M.O. is to agree in the meeting and then send a contradicting email afterward."