It's when an extrodinaraly old man takes viagra and it gets bigger, but not hard and flops around like a trout, then he slaps a girl in the ass with it.
Phil's dad gave me a rusty trout last night
The state of a vagina from having anal sex and then switching to vaginal sex, thus "rusting" up the trout.
I was so horny this morning that I ate my wife's rusty trout leftovers from last night.
The art of going fishing then coming back home and sticking a trout up your ass then eatting it or sticking it up some one else's ass.
Nick gave his whole family a rusty trout for dinner after they got home from fishing and shrimping