Brewer's: Old Boots
Like old boots. Famously. “Cheeky as old boots,” very saucy. “He ran like old boots,” i.e. very fast. The reference is to the nursery story of the Seven-leagued Boots, old being simply a word of fondness, as “Well, old boy,” etc. The allusion, suitable enough in many phrases, becomes, when used in slang, very remotely applicable.