"and sent him half a guinea under the seal"
George lll Half Guinea, dated 1786
GNU Free Documentation LicenseGeorge lll Half Guinea, dated 1786 - Credit: Classical Numismatic Group, Inc. http://www.cngcoins.com

 The guinea was a gold coin minted in Britain between 1663 and 1813. During its period of usage, its value fluctuated between 20 shillings (one pound) and thirty shillings. Following the Great Recoinage of 1816, the guinea was replaced by the sovereign. However, the term guinea was used right up until the decimalisation of the British currency in 1971 to indicate a value of 21 shillings. Typically, it was used to suggest that something was cheaper than it was, as eight guineas (for example) sounded less expensive than eight pounds and eight shillings.

The half guinea gold coin was introduced in 1669 and was also discontinued in the Great Recoinage of 1816.