Probably from "chance" meaning "a random happening."
Perhaps the character is named after Chauncey Gardner or "Chance the Gardener" from the 1979 satirical film "Being There," based on the novel of the same name by Jerzy Kosinski. British actor Peter Sellers played the enigmatical Chauncey, a man who is basically a simpleton, but who sounds like a genius to everyone he meets.
The Famous Wizard card states that he was the first known victim of Dragon Pox. However, Gunhilda Kneen in the 1100s had to sit out a game of Quidditch with a case of Dragon Pox (QA3).
Oldridge was Rowling's "Wizard of the Month" for July 2005.