This is a more inside way of saying you have to
drop a deuce (
take a crap), it works really well in an office setting so that you may let your close, personal co-workers know where you are going without letting everyone else know. This is derived from John Wayne's nickname being "The Duke", and can be shortened by simply saying "I have
to make a phone call", or added to by saying "man John Wayne has been calling me all day."