To throw up. Most commonly associated with excessive ingestion. Throwing up when you have the flu would not be booting. Throwing up after eating or drinking too much would be.
A 'Boot' is a generic word for someone who is an idiot or un-attractive.
Providing you were straight:
You would generally call a fellow member of the same sex a 'Boot' for being stupid/an idiot.
You would call a member of the opposite sex a 'Boot' if they were un-attractive.
If you are not straight it would be the other way around! Both definitions could be used to describe the same person.