stuff up verb 3stuff-up noun 1forms 4 verb3 •To cause (the nasal passages) to be blocked.My nose is all stuffed up so I'm speaking funny. •(slang) To ruin or harm.Don't drink tonight, because it'll stuff you up before your business trip. •(UK, slang) To make a big mistake.We lost the match because of the goalkeeper stuffing up. stuff-up noun1 •A mistake or fail. formsstuffed up · stuffing up · stuffs up · stuff-ups