A Malaphor is when two common idioms or cliches get mixed up, producing a nonsensical and often amusing hybrid. The word is generally ascribed to Lawrence Harrison who wrote a piece 'Searching for Malaphors' in the Washington Post August 6 1976. Malaphors often have a kind of weird surreal sense of their own
A couple of classic examples will give the flavour.
'We'll burn that bridge when we come to it'
'He smokes like a fish'
Here's a new one I heard a couple of days ago
'That's all spilt milk under the bridge now'