Have you ever been invited to a birthday party that you don't want to go to ?
Bought a top and then thought.. why on earth? and had to pretend it was from your Grandmother who is a bit batty, and colour blind etc
Think of an excuse... an easy simple excuse.. less explanation is more realistic, an elaborate storyline looks like just that, a story, so keep it simple.
Other demands, conflicting appointments, you could even tell them the truth that you are deferring your paper as your work requires further crafting. I would disagree with your sup though, ignoring just seems like bad manners to me.. whereas lying, white-lying, is definitely ok.
Chuff