Spam: After the Monty Python sketch with Vikings in a cafe singing about spam, the word became associated with needless repetition of messages. It eventually migrated onto newsgroups, then eventually onto the real internets.

Using 'gay' to mean 'bad' or 'abnormal' is just a regrettable extension of the old belief that homosexuality is an inferior abnormality. Just like 'queer' used to be a derogatory term for a homosexual man, as 'queer' means abnormal, malformed, unusual, etc. When someone says 'your shirt is totally gay', they mean either that it looks like something a gay person would like, or they mean that your shirt is inferior, emasculating, weird, and not in accordance with the majority.

Most of these word associations come about for a reason, though the reason can be lost pretty quickly sometimes, if it's even obvious at the start.

Personally, I'd like to know why snowboarders think that favourable conditions are 'sick'. It sounds so idiotic to me. "My sick was really fat!"