Jock male/female chauvinists fantasizing over fannies(butts) in ghetto street talk fashion. A blending of the words "fantasizing" and "fanny" creating "fannysize". Any play on the word fantasy that leans to anything sexual.
Ted says to Harry regarding Gina's bum,
"I'd like to drive it home". Harry replies, "keep on fannysizing!"
Fannytime's Day is the English equivalent of Valentine's Day - Fannytime’s Day occurs every February 14. In England, chocolates, flowers and gifts are exchanged between loved ones, all in the name of St. Fannytime.