Someone who doesn't finish his drink. i.e. someone who leaves an half inch of swill at the bottom. I've heard this term in Buffalo, NY. and may be named after someone but no one knows anyone named Gillety.
Pronounced like the shaving brand gillete with a Y sound at the end.
"Are you gonna finish that beer or are you going to be a gillety."
"when cleaning up after the party I realised my friends were a bunch of gilleties"