chipmunk baby

A child conceived through clandestine means on the female's part. Said means usually consist of the female giving fellatio, pretending to swallow, concealing semen in the side of her mouth (as a chipmunk), and later transferring the semen from the mouth to the vagina.

"... but Steve told me last year that they didn't want to have kids! I bet that sneaky bitch Jill made herself a chipmunk baby."