Green’s Dictionary of Slang

1. (US) a $20 bill [double ten].

1946		J.E. Dadswell Hey, Sucker 87: Dub (meaning double sawbuck) for twenty dollars.
2000		W. Shaw Westsiders 168: ‘Three out the dub.’ He’s won £3 dollars out of the double — the $20 note.

2. (N.Z.) a double-decker tram.

1954

Parson’s Packet Mar.–Apr. 3: So that our next-to-eldest can get away [...] to catch the ‘8 o’clock dub’. The double-decker tram just have been on the tracks for over fifty years [DNZE].

3. the number 20 [double ten].

2000

W. Shaw Westsiders 265: Rah just made the ‘dub one’ — he is 21 years old.

4. (US drugs) $20 worth of a given drug.

2015

Chief Keef ‘Laurel Canyon’ 🎵 Aye, pull up get that dub then I get back.