"Hay penny" is a variant spelling of "halfpenny," a coin used in the United Kingdom, according to the Collins English Dictionary. The Royal Mint Museum notes that halfpennies were regularly used as far back as the 13th century, but BBC News says the coin was phased out in 1984.
The halfpenny was the smallest coin used in the United Kingdom. It was worth half of a penny, but by the time it was abolished, the BBC News reports that very few people used the coin. The coin's removal from production was announced by Chancellor Nigel Lawson, and the Royal Mint ceased creating halfpennies in February of 1984. The coin was no longer considered legal tender in December of 1984, says BBC News.