Green’s Dictionary of Slang

1. (Aus. drugs) a foil-wrapped package of heroin.

1997

L. Davies Candy 91: An unsuspecting (real) customer had bitten into a nest of foilies in the middle of his cheeseburger.

2. (US drugs) a measure of a narcotic poured into kitchen foil, heated until smoking and then inhaled.

2006		Urban Dict. 2 Oct. 🌐 foilies [...] The finished preparation used to smoke freebase cocaine. An amount of cocaine is layered somewhat thinly on a foil sheet, baking soda is added in approximately half the amount of cocaine. A drop of water added, dried, then smoked in a ‘chasing the dragon fashion’.
2012		M. McBride Frank Sinatra in a Blender [ebook] He didn’t know what to do first — look inside the bag of money or smoke a foily [...] He emptied most of the baggie on the foil.
2014		M. McBride Swollen Red Sun 123: They polished off another foily.

3. see foil n. (1)