muck-up noun 2muck up verb 2forms 4 noun2 •(informal) The act of botching something; an incompetent mistake. •(Australia, informal) A prank. muck up verb2 •(informal) To ruin unintentionally.The terrorists have really mucked up our holiday plans. •(Australia, informal) To clown around; to have fun, often at the expense of others.It's the last day of school and everyone is mucking up. formsmucked up · mucking up · mucks up · muck-ups