"CAT+UP" ("catsup") -- is a joke sign for "ketchup."  It started in the Deaf schools and spread throughout the Deaf community. Thus one of the signs for ketchup is indeed "CAT+UP." 
Of course, lots of people just mime "hitting the bottom of an upturned bottle" to mean ketchup but I think it is good for students to see "real life" signing as done in the Deaf world -- and that includes the puns.