An ignoronicism is a portmanteau of the words "ignorant" and "ironic." It refers to an occasion where an individual misspeaks a phrase out of ignorance of the intended phrase, specifically with the result being that, ironically, the opposite meaning is issued. Ignoronicisms are like malapropisms, but contain an added ironic layer in that the listener's perception, taken literally, is that the speaker means the opposite of his or her intention.
For example:
"He never seemed to amaze me." In this case, the speaker should say "ceased to amaze me," and, through ignorance of the correct phrase, states that the subject never amazes.
"I could care less" Here, the speaker intends to state the correct phrase, "I couldn't care less," but in ignorance, states the opposite.
"Robbing Peter to play Paul." The phrase "Robbing Peter to pay Paul," implies that the speaker rewards Paul. In this ignoronicism, the speaker actually states that he seeks to "play," or fool, Paul.
"She's a real knocked out." The subject is, according to the accurate phase, a "knock out." However, our speaker inadvertently places her as the one who is knocked out.
Ignoronicisms generally occur when a speaker hears and repeats a commonly-used phrase without understanding the full articulation of the phrase. In most audiences, however, the context of the phrase is enough to effectively communicate the speaker's intended meaning.