While he's in the cell, Black Widow approaches Loki and asks him to stop mind-controlling Hawkeye. The Norse god responds to this by ingesting 20 cans of Alphabetti Spaghetti and going on a verbal diarrhea monologue where he insults and taunts Black Widow about her pathetic sentimentality before ultimately calling her a "mewling quim." If you don't know what a "quim" is, then you and my spell check have at least one thing in common. But a quick Google search reveals that the word is actually a British synonym for "female genitalia," specifically the term for it that begins with a "C" and rhymes with "Hold on, did Loki just call Black Widow a 'c**t?!'"
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No, he called her a "whining c**t." Please pay attention.
The best thing about this sneaky cursing is probably how fitting it was for the character delivering it. After all, Marvel's Loki is kind of a dick who sounds like an angry Victorian gentleman getting a subpar handjob during his daily dictionary reading, so it'd actually make sense for him to drop a "sophisticated" British C-bomb. No wonder then that Whedon considers getting that line past the censors to be his "greatest achievement." Sort of like how my greatest achievement was secretly hyperlinking a picture of gay horse sex in the second "o" of "foot fetish" in the second paragraph.