Nadsat for the testicles, or scrotum, or bollocks.

The origin of the word appears to be the Russian word yabloko, meaning `apple'.

Best used in the insult

"Come and get one in the yarbles, if ya have any yarbles, you eunuch jelly thou"...

(from Anthony Burgess' novel A Clockwork Orange and expressed perfectly by Malcolm McDowell in Stanley Kubrick's film version)

Infinite thanks to Frater 219 for additional information.