A malapropism is the substitution of a word with a similar-sounding, incorrect word.  For example, people always say “for all intensive purposes.”  This doesn’t make any sense.  It’s supposed to be “all intents and purposes.” 

If anyone hears me using a malapropism, just cut my head off.  I know cutting my head off seems extreme, but I give you full permission to decaffeinate me.