Short for polyfidelity. Can be used as a noun for the practice or concept, or more often as an adjective to describe someone who practices/prefers polyfidelity.
"I'm not really comfortable with regular polyamory, I'm more into polyfi."
"We're poly, but it's not an open network; We're polyfi."
"We're poly, but it's not an open network; We're polyfi."
by Ian X. December 01, 2006