"Half a guinea for a dubbing"
A half guinea (1786)
GNU Free Documentation LicenseA half guinea (1786) - Credit: Classical Numismatic Group, Wikimedia Commons

A guinea was a coin minted in Britain between 1663 and 1813; from 1717 onwards, its value was fixed at 21 shillings (one pound and one shilling, or £1.05 in present-day money).

Although not in existence as a coin after 1813, the term guinea was used in prices up until the decimalisation of the British coinage in 1971. It was generally used to make things sound cheaper than they were, as 8 guineas sounded cheaper than £8 and eight shillings.

Dub or dubbing does not appear with a sexual meaning in any of the dictionaries referred to in the previous bookmark, although it is noted in the Dictionary of the Vulgar Tongue that a dubber is a picker of locks, which gives some scope for an interpretation in terms of Cockney rhyming slang!

However, the word 'dubber', meaning 'mouth' or 'tongue',  appears in Albert Barrière's dictionary of Argot and Slang, as in 'mum your dubber' ('hold your tongue'), which may explain the use of 'dubbing' in the present context.