phallapropism

The incorrect use or interpretation of a word pertaining to the male genitalia resulting in the substitution of one with a similar sound (viz. malapropism).

"You seen the size o' that ole boy's dong?" asked Cletus, staring bitterly down at his own withered package.

"I ain't interested in how big a turd Jimbo can lay," replied Larry. His phallapropism was no surprise; he'd always had trouble distinguishing between soft vowel sounds.