A colloquialism of British origin, specifically from the north-east of Scotland, which is used in conversation as a reference to the act of engaging in anal sex.
When a vagina is gaping so widely one can fit a trundle bed within it and house multiple inhabitants.
When Amanda and Gregg discussed the sex life of their friend, jessica, they said "her vag is such a trundle fuck, of course she can fit her husbands dick and her part time lovers in at the same exact time!!"