Flagging

Flag meaning “cloth ensign” is first recorded in English in the late 15th century and is “of unknown origin,” but is likely connected to flag (v.1) “flap about loosely,” from Old Norse flaka “to flicker, flutter, hang loose” — an apt image for the little hanging tab on a mechanical clock. In chess the flag is literal hardware: a small indicator, usually red, on an analogue chess clock that falls to indicate the exact moment a player's time has expired. To “flag” someone means winning on the basis of the opponent exceeding the time control. The device is datable: the flag mechanism appeared at the turn of the 19th century, helping settle chronic arguments over when a player had exceeded a time limit.

First documented: Flag mechanism on analogue chess clocks, dated to “the turn of the 19th century”