Definition from Wiktionary, the free dictionary
Uncertain; perhaps most likely bog (“toilet”) + standard with obscure semantic development. Believed to derive from early ceramic toilets being produced to exactly the same specification in white only which became known as the bog standard.  
- (idiomatic) Especially plain, ordinary, or unremarkable; having no special, excess or unusual features; plain vanilla
- She drives a bog standard economy car.
- See also Wikisaurus:common