slip up verb 2/ˈslɪpʌp/ noun 1forms 4 verb2 •To lose ones footing; to slip. •To err, falter; to make a mistake, especially a seemingly small error.I hope I don't slip up during my presentation.Somebody slipped up and forgot to put your name on the guest list. slip-up /ˈslɪpʌp/ noun1 •A (small) error or mistake; a (minor) misstep.I think she will forgive an accidental slip-up, so don’t worry too much about misspeaking. formsslipped up · slipping up · slips up · slip-ups