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."
From the Latin root "rubius", meaning small brain. And from the Russian "rubanikov", meaning stone drunk. Also questionable Ehtiopian origin, indicating a state of lethargy induced by narcotics.
Modern Definition: A failed attempt at humor, often perpetrated by he who is named "ruby". Syn: Retarded
Dude, did you see the teacher pull a Rubyism?
Yeah man, it was retarded.
Just like that Rubykid.