slang word with several, mostly insulting
1. Pejorative term for a member of the Irish traveling community. This still remains the main usage, but because of the traits (unfairly) associated with these people it leads to...
2. Anyone who is either shifty and untrustworthy, or who is unclean and has generally bad personal hygiene. Travellers are viewed as smelly thieves, which is why those characteristics are associated with the word.
3. To break something. This use is also common in Britain - "my car is knackered", "now Shearer is injured, Newcastle are knackered", and so on. Can also be strectched to mean exhausted, as in "I've been up for 36 hours, I'm bloody knackered". It's a reference to the Knancker's Yard, a place where old, tired horses were retired.
4. Testicle. May only be used if you are a cast member of Monty Python or Bottom.