It probably started as a rugby league chant in the northwest of England where pies are traditionally sold before during and after matches. The chant was, and still is, levelled at anyone appearing to be overweight. Often directed at a player, who is bulky, to whom fans want to express dislike or good humoured heckling. Possible origin was at Wigan RLFC where Wigan folk are known as the pie eaters. This apparently originated in the 1920s when the Wigan miners returned to work after a strike having declared that they would never do so ... thus having to eat "umble pie"