Malapropism happens when a speaker mistakes a word to create comedy. Usually the words have multiple syllables and sound alike but are different in meaning. Dr. Wheeler gives the example of when a maid falls into the river in Tarzan of the Apes and exclaims, "I sho'nuff don't want to be eaten by no river allegories, no sir!" (Dr. Wheeler, "Literary Terms and Definitions").
One example of malapropism in Twelfth Night can be found in Act 1, Scene 5. Sir Toby has come to report that there is a messenger at the door, and Olivia is absolutely repulsed to find that he is already drunk so early in the day, as we see in her line, "Cousin, cousin, how have you come so early by this lethargy" (115-16). By "lethargy," she is referring to the drowsiness or dullness that's a symptom of drunkenness; however, Sir Toby mistakes it, either intentionally or unintentionally, for the word "lechery," meaning "lewdness," or excessive sexual desires, as we see him state in his declaration, "Lechery! I defy lechery" (eNotes, 117).
A malapropism is a comic substitution of one word for another or a word mixup that is meant to raise laughs. An example of a malapropism occurs in Twelfth Night in the first act, scene three, when Sir Toby Belch mixes up the words subtractors and detractors. After Maria puts down his friend Sir Andrew Aguecheek, Sir Toby responds:
They are scoundrels and subtractors
That say so of him.
The "s" in subtractors makes for better alliteration but, in fact, people who put other people down are called detractors. Sir Toby drinks too much (Belch is an apt surname) and people who drink tend to mix up their words.
Sir Andrew also engages in malapropisms: he mistakes Sir Toby's meaning when Sir Toby tells his to "accost" Maria, thinking accost is her name. Sir Andrew greets her as "Good Mistress Accost." Of course, what Sir Toby means is that Andrew should make sexual advances towards Maria. When Maria corrects Sir Andrew by saying her name is Mary, her repeats the mistake, this time calling her Good Mistress Mary Accost.
These malapropisms help characterize Sir Andrew and Sir Toby as the comic fools they are in the play.