parent intrusion flag; parent alert
Internet slang used by streamers to signal that a parent has entered the room, often prompting a change in behavior or topic.
A parent flag went up during the stream, so I suddenly switched to polite speech.
A warning about a parent flag appeared in the chat.
Compound of 親 (parent) and フラグ (flag), from English 'flag' in the sense of a signal or event trigger. The term emerged in Japanese streaming and online communities.