After a female as lightly excreted feces on her partner's genitalia, she strokes the scrotum in a petting like fashion. To make this a Rusty Pineapple though, there must be a "five o'clock shadow" on the scrotum, to create a pineapple-ish effect.
The fetus baby said that he smelled a Rusty Pineapple in the vicinity.
"DUDE. I got a Rusty Pineapple last night and now I feel violated"