Green’s Dictionary of Slang

poly n.

also polynesian
[abbr. SE Polynesian, generic for a far-away place]

(UK drugs) marijuana of no particular origin or quality.

[UK]N. Barlay Curvy Lovebox 24: Polynesian’s what Nood says he has when he don’t know what the fuck he does have. [Ibid.] 33: There ain’t nuthin’ boom about dis poly.