A malapropism (from French mal à propos, "ill to purpose") is an incorrect usage of a word, usually with comic effect. The term comes from the name of Mrs Malaprop, a character in Richard Brinsley Sheridan's comedy, The Rivals (1775), whose name was in turn derived from the existing English word malapropos, meaning "inappropriately". Here are some examples from her dialogue:

"He's as headstrong as an allegory on the banks of the Nile." (i.e., alligator)

"He is the very pineapple of politeness." (i.e., pinnacle)

"If I reprehend any thing in this world, it is the use of my oracular tongue, and a nice derangement of epitaphs!" (i.e., apprehend; vernacular; arrangement; epithets)

However, it might be more appropriate to call such confusions "Dogberryisms" after Sergeant Dogberry in William Shakespeare's Much Ado About Nothing, who was making them almost two centuries earlier, for example:

"Comparisons are odorous." (i.e. odious)

"Our watch, sir, have indeed comprehended two auspicious persons." (i.e. apprehended; suspicious)

A malapropism found in a student paper reads:

"Many people are morally opposed to intravenous fertilization." (i.e. in vitro)

Common malapropisms in modern English include use of:

• Disinterested (impartial, unbiased) for uninterested ("A judge should be disinterested, but not uninterested")
• Fortuitous (random, by chance) for fortunate
• In the ascendancy for in the ascendant ("One has the ascendancy" vs "One is in the ascendant")
• Barbaric for barbarous ("Barbaric" can be positive and is used of culture, "barbarous" is negative and used of behavior: "Barbaric splendor" vs "Barbarous cruelty")
• Enormity (a heinous act) for enormousness (being very large).
• Noisome (smelly) for noisy (very loud)

Ringo Starr was famous for his many malapropisms which became Beatles songs:

• Tomorrow never knows
• It's been a hard days night
• Eight days a week

See also:

• Fowler's Modern English Usage
• Pleonasm
• Elegant variation
• Yogiisms