Why is 5 pence called a bob?

The term 'bob' for 5 pence originated from the British currency system before 1971. At that time, a shilling was worth 12 pence, and a shilling was often referred to as a 'bob'. After the decimalization of the currency, the new 5-pence coin was equivalent to the old shilling, and the term 'bob' continued to be used for it.