A short phrase (with variations such as "i'm pretty tired") used as a simple explanation when a person doesn't want to do something but won't tell you the actual reason as to why - usually used as a replacement for "i can't be bothered" or "i don't want to". Also used as a form of rejection without openly saying it.
A: Hey man, you still coming to the pub tonight?
B: Nah.... i'm feeling pretty tired now.
A: You woman...
Guy: Hey.. d'you want to go out for dinner tonight?
Girl: Um.. nah.. I'm tired.. sorry
Guy: Oh, well, how about tomorrow?
Girl: No.. I.. have to wash my hair...