"What Was He Cooking?" is a reaction image and catchphrase built from a YouTube thumbnail by film-analysis channel New Rockstars2, showing Thanos apparently at a stove during an Avengers: Endgame scene next to the bold yellow caption "WHAT WAS HE COOKING?"1.

The wording works as a punchier stand-in for "what was he thinking?", typically deployed when someone makes a spectacularly baffling error. It sees especially heavy use with Super Smash Bros. Ultimate clip-sharing accounts, where "what was he cooking?" punctuates footage of players throwing away stocks on bizarre inputs or misreads3.

The format typically works as a one-line reply to a clip or screenshot where someone makes a strange, self-defeating choice4. Posters often drop the Thanos thumbnail under the offending post directly, or type "what was he cooking?" as a caption on the clip itself5.