Quoted By:
Actually, it's a slang pun.
His name is "Masutādo" (マスタード) which is Engrish for mustard, but also
it's like Master (マスター) + Dojo (ドジョー)
but they cut off the "jo" (ジョー).
The pun doesn't really translate back into English well, so he's just Mustard for us.
