Green’s Dictionary of Slang

hundred n.

1. (Aus. drugs) A$100 worth of heroin.

[Aus]L. Davies Candy 7: The phone rings and it’s Micky Fleck wanting a hundred.

2. (N.Z. prison) a 100 mg morphine sulphate tablet.

[NZ]D. Looser Boobslang [U. Canterbury D.Phil. thesis] 91/1: hundred (also hundred m) n. a 100mg morphine sulphate tablet.

In phrases

keep it a hundred (also keep it 100)

to keep it honest, ‘keep it real’ [SE one hundred per cent].

[US]Woods & Soderburg I Got a Monster 101: ‘I don’t care about the gun or the drugs. You wanna go somewhere and talk and keep it a hundred or not? Last chance’.
[US]Woods & Soderburg I Got a Monster 146: ‘Yo my nigga,’ he wrote. ‘I got some real 100100 shit to say. [...]. Ain’t that many 100100 niggas out here.’.
one hundred (adv.)

(UK black) definitely, without any doubt [SE one hundred per cent].

[UK]G. Krauze What They Was 224: I’m a bell you back later one hundred.