Mastodong Posted in M Best definition Mastodong Originally derived from the word Mastodon, this term describes an enormous penis. Bob: Fuck man it was a total slayfest last night Me: Shit Bobs got that Mastodong! How satisfied are you with the definition? Looking for Mastodong meaning? Delighted that you discovered the greatest determine on our web site! Recently this definition was actually observed 69 times.