Why Black Widow Is So Scared of Hulk in 2012's Avengers - Their Secret MCU History Explained 

screenrant.com -BY ROBERT WOOD -When Black Widow meets Bruce Banner in The Avengers, she's terrified - because she already faced the Hulk earlier in MCU continuity.
Keywords: black widow, scared, hulk, 2012, avengers, secret mcu history, explained, natasha romanoff