In UK slang terms the "dog's bollocks" means the best. It comes from the habit of male dog's licking their testicles (bollocks) and that they must taste good as they spend so much time in this activity.

British slang meaning one of the following.

1) To be the best example of its kind.

2) Dog testicles.

3) An alcoholic drink that can be found somewhere in most pubs or bars in the UK.

of outstanding quality
originating from 1950s packaging
box, deluxe - dog's bollocks
box, standard - bog standard

the best thing ever, not to be confused with bollocks!

British Slang for 'the best ever'
also:
1.literally - dogs testicles
2.a game played on school playgrounds in Britain similar to British Bulldog but more hardcore

(UK) Slang for something that is the cremé-de-la-creme, the leader in it's field

According To the Early Twenty-First Century Commedian Eddie Izzard, The 'dog's bollocks' is a good compliment, as opposed to 'bollocks' which is a bad compliment.