It's part of the comics lexicon familiar to manga and manhwa readers: an inverted word-balloon tail indicating that the speaker is off-panel.

Inverted tail
An inverted word-balloon tale indicates that a character not shown in the panel is speaking. It's common in manga and manhwa but much less frequently used in American and European comics.
Should you use it?
Keep reading with a 7-day free trial
Subscribe to The Webcomics Handbook to keep reading this post and get 7 days of free access to the full post archives.