An exclimation made by a man who is certain that he is about to recieve some kind of intercourse, be it a cheeky blowy or full on balls deep poon pounding.

The words are usually said with assonance of 'choo choo' (like a train') and the arm movement commonly seen when a train driver is honking a horn

A vagina. A poontang. A beef curtain. A cunt. A twat.