Green’s Dictionary of Slang

1. of an engine, a vehicle, to accelerate.

1904		Kipling ‘Their Lawful Occasions Pt I’ in Traffics and Discoveries 117: Can we whack her up to fifteen [i.e. knots], d’you think?
1914		‘Bartimeus’ ‘The Ship-Visitors’ in Naval Occasions 46: They did not hear the hoarse whisper conveyed down the voice-pipe to the Leading Stoker to ‘Whack her up!’ And so they failed to realise that they were throbbing through the water at a speed which [...] was exceedingly bad for the engines and wholly illegal.

2. (Aus. prison) to inject narcotics.

1990

Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Whack-up. 1. To inject narcotics.