Up until around the age of 12 this is a common phrase in every child's vocabulary. Used to question the validity of somebody's statement. In cambridgeshire there were two variations of chinny reckon or "Chinny Rah-Kon". Firstly one would put their tongue imbetween their bottom teeth and lip, making it swell, and make a low noise before saying "Chinny Reckon" or "Itchy Chin". The phrase "Chin Spin" was also used in the same context.
"I've scored twenty five goals on the playground this week"