A yegg is defined as being an agile burglar. This means the same as a cat burglar, housebreaker, or cracksman.

robber, burglar, housebreaker, shoplifter, pickpocket, purse snatcher, sneak thief, embezzler, swindler, criminal, crook, bandit.

hamburger burglar, meat-o bandito, veal-er stealer, sirloin-er purloiner, steak-er housebreaker, meat packer safecracker

A cat burglar is not a real cat. These are the definitions of a cat burglar:

Noun - A burglar who is especially skilled at stealth or undetected entry of a premises. (Hence the name 'cat burglar')

Noun - A burglar who enters buildings by climbing through upper windows, skylights, etc.

(cat) burglar (go down the) gurgler