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
by Stopher Reinhold September 27, 2006
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.
by Mark of Pembrokeshire February 09, 2009