A form of self-censorship practiced at work to avoid offensive or cuss words. Typically includes cuss-replacements you learned from your grandma. Potentially embarrassing if accidentally used outside of work at parties or in the company of your drunk friends.

May also be used in the company of grandparents, teachers, preachers, and others who disapprove of cussing.

Does not apply to all professions. Musicians and construction workers have no need for a work mouth.
> At work:
> -Did you just say fuck?
> -Yeah, sorry. I forgot to use my work mouth.
> At a party:
> -Did you just say fiddlesticks?
> -Yeah, sorry. I still have my work mouth on.