Is it ensure, insure, or assure?
student slang, "dolt, idiot, nerd," by 1960s. "Dictionary of American Slang" says "probably related to doo-doo and goofus."
: Many another dufus play among friendsnoun
A fool; idiot; airhead, birdbrain, boob: He'll do his best to make you feel like a doofus/ But this is the doofus you have to deal with, so hush up/ I have to be in front of this self-important doofis with his portable phone/ I felt like such a dufus/ when some big, loud, popcorn-chuggin' doof and his date sit in front of me
[1960s+; probably related to doo-doo and goofus]