English Preposition inside of Inside Return it inside (of) a month. 1954, Jim Brewer, Boys’ Life, volume 44, number 2, Think and Grin, page 78, column 1:A book is a man’s best friend outside of a dog, and inside of a dog, it’s too dark to read. References “inside of”, in OneLook Dictionary Search. Anagrams sinofied, sonifiedWikiwand - on Seamless Wikipedia browsing. On steroids.