stern
n.n. serious and strict in a way that shows you are not joking. You use this to describe someone's face or voice when they are being very firm.
n. serious and unrelenting, especially in the assertion of authority and discipline. Often used to describe facial expressions or vocal tones that convey disapproval or gravity.
The teacher gave the students a stern look.
The manager issued a stern warning to the staff about arriving late for their shifts.
Despite his reputation for being a stern disciplinarian, the judge was known to show occasional flashes of dry humor during particularly long trials.
* As a German and Jewish surname, from the noun Stern (“star”). This was also borrowed into various Slavic languages such as Serbo-Crotian and Slovenian. * As an English surname, from the adjective stern.
Typically precedes the noun it modifies; frequently collocates with 'warning', 'look', or 'face'.