idioms involving “flag”

A red flag

A warning signal

“The sudden drop in sales was a red flag that we shouldn't have ignored.”

Flagging

Waning, dwindling

“The sales of our top brand are flagging so we must increase the advertising budget.”

Flag down

Signal, get the attention of

“Please see if you can flag down a taxi while I finish locking up the office.”

Flag waver

A very patriotic person

“The flag wavers were marching to show their support for the troops.”

Run something up the flagpole

To mention an idea to assess the reaction

“I like your idea, Higuchi-san. Let's run it up the flagpole in next Tuesday's meeting and see if it flies.”