Any analogy, metaphor, etc where the person speaking/writing is attempting to use a common phrase but messes it up by either mixing multiple common phrases, or simply replaces key words in the phrase with incorrect words. The most important aspect of a ruddism is that they are not done on purpose. The person must think they said it correctly.
Examples of a ruddism:
"A bird in a hand is better than two bushes."
"You have enough to do. I don't mean to throw more worms on the fire."
"She was green with ivy."
"You can't teach a dog old tricks."
"That's just my cross to burn."
"You can lead a gift horse to water, but you can't look it in the mouth."
"Six of one. Half of the other."
80's British anarcho band fronted by madman Nick Blinko. Known for their obscurity, misanthropic viewpoints, rare performances, and distinct rawsound. The general atmosphere of their output is generally regarded as very bleak, depressing and hopeless, though they often make very prolific social statements.
Matthews explains how he came up with the name of the band: "When I was at school studyingbiology, we were told that in the fetal stage the clitoris is a rudimentary penis."