From the "slang money words, meanings and origins glossary"
bob = shilling (1/-), although in recent times now means a pound or a dollar in certain regions. Historically bob was slang for a British shilling (Twelve old pence, pre-decimalisation - and twenty shillings to a pound). No plural version; it was 'thirty bob' not 'thirty bobs'. Prior to 1971 bob was one of the most commonly used English slang words. Now sadly gone in the UK for this particular meaning, although lots of other meanings remain (for example the verb or noun meaning of pooh, a haircut, and the verb meaning of cheat). Usage of bob for shilling dates back to the late 1700s. Origin is not known for sure. Possibilities include a connection with the church or bell-ringing since 'bob' meant a set of changes rung on the bells. This would be consistent with one of the possible origins and associations of the root of the word Shilling, (from Proto-Germanic 'skell' meaning to sound or ring). There is possibly an association with plumb-bob, being another symbolic piece of metal, made of lead and used to mark a vertical position in certain trades, notably masons. Brewer's 1870 Dictionary of Phrase and Fable states that 'bob' could be derived from 'Bawbee', which was 16-19th century slang for a half-penny, in turn derived from: French 'bas billon', meaning debased copper money (coins were commonly cut to make change). Brewer also references the Laird of Sillabawby, a 16th century mintmaster, as a possible origin. Also perhaps a connection with a plumb-bob, made of lead and used to mark a vertical position in certain trades, notably masons. 'Bob a nob', in the early 1800s meant 'a shilling a head', when estimating costs of meals, etc. In the 18th century 'bobstick' was a shillings-worth of gin. In parts of the US 'bob' was used for the US dollar coin. I am also informed (thanks K Inglott, March 2007) that bob is now slang for a pound in his part of the world (Bath, South-West England), and has also been used as money slang, presumably for Australian dollars, on the Home and Away TV soap series. A popular slang word like bob arguably develops a life of its own. Additionally (ack Martin Symington, Jun 2007) the word 'bob' is still commonly used among the white community of Tanzania in East Africa for the Tanzanian Shilling.