Chunder means to be sick, it originates from old seafareing days when sailers would get seasick and stick their head out of the porthole in their cabin. As they did this they would shout "Watch Under" to warn people in lower cabins of the forthcoming puke. Over the years this has evolved in Chunder.
Throwing up after a heavy drinking session, either on the evening or the following morning. Chunderbirds involves a ranking system dependant upon the amount of time thrown up.
Chunderbird private (1-2)
Chunderbird sky officer (3-4)
Chunderbird ground warden (5-6)
Chunderbird brigadier (7-8)
Chunderbird death agent (9-10)
Chunderbird apache warrior (11+)
When about to throw up, or after someone has thrown up. Chunderbirds are GO!
Referred to when a female (or male) has been out on the lash and begins to chunder. She is sitting down with her legs apart in the same way a woman would sit when giving birth and is being sick between them. As if the chunder is her new born baby.
Girl: I was sat on the street puking between my legs on the road I think... very classy
Dude: Sounds like you had a Chunder Baby last night!