“Hey, that’s not cool” is four words. Most people go whole years without saying them.

Objecting out loud is almost never a matter of knowing what to say. Everyone knows. What stops people is that saying it makes you briefly the odd one in the room, and the silence afterwards lasts a second longer than feels comfortable. So it goes unsaid for years by people who all privately disagreed, and the thing continues because nobody paid that one second.

Have a short line ready so you are not inventing one under pressure, and keep it flat and unaggressive. Use it once this week on something small. The cost is lower than you think and the room usually moves with you.