Mrs. Malaprop is a comic character in Sheridan's The Rivals. The most notable thing about her is the way she uses incorrect words to express herself, words that sound similar to the appropriate word but which have a completely different meaning. Mrs. Malaprop's struggles with the English language gave rise to a new literary term—malapropisms, of which there are numerous examples in The Rivals. To name but two:
The pineapple of politeness. [She really means the pinnacle of politeness].
She's as headstrong as an allegory on the banks of the Nile [She's thinking of an alligator].
Mrs. Malaprop's verbal gaffes undercut her status as an authority figure in the play. She's a rather pompous, self-important woman, forever dispensing matronly advice to her niece Lydia. Yet her ignorance of the very words she speaks means that she can't be taken seriously. She's also more than a tad hypocritical, as the following line makes painfully clear:
I would have her instructed in geometry, that she might know something of the contagious countries;—but above all, Sir Anthony, she should be mistress of orthodoxy, that she might not mis-spell, and mis-pronounce words so shamefully as girls usually do; and likewise that she might reprehend the true meaning of what she is saying [emphasis added].
Here is Mrs. Malaprop using a malapropism to warn against the dangers of malapropisms. Self-awareness is evidently not one of her strong points.
Sheridan is making a rather subtle satirical point through the character of Mrs. Malaprop. It says something about so-called respectable society that someone as ignorant and as ludicrous as Mrs. Malaprop can presume to exert moral authority over others. Sheridan also seems to be suggesting the need for a broad, systematic education for those who would engage in polite society. Mrs. Malaprop, like the hapless Captain Le Brush, lacks the benefit of a decent education, and the consequences are painfully obvious each time they open their mouths.