after meat, mustard
A phrase emphasizing that something has come too late to be useful (like mustard after meat has already been eaten). A: "Here, I finished my project, Mr. Smith!" B: "Ah, after meat, mustard. I've already submitted your final grade, and it's too late to change it."
the pope's mustard maker
A pompous person, especially one in an insignificant job or role. In the 14th century, Pope John XXII had a personal "moutardier" (mustard-maker), to ensure that his food was properly seasoned. All she does is copy files all day, but she acts like she's the pope's mustard maker. I practically have to beg for a chance to use the copier!
be as keen as mustard
To be very enthusiastic about something. Primarily heard in UK, Australia. These new kids are as keen as mustard to be in the choir, so we can definitely get them to clean up the choir room for us.