Has a couple of other meanings, of course: to start your stopwatch, and to attack. I was doing a bounce sortie against a Harrier 4-ship many moons ago and, since the radar was u/s, I asked the Harrier leader 'Can I have a hack at the ridge?' to help the interceptions. Nothing happened for the rest of the afternoon, as I waited for his 'time' hack at the ridge and he waited for my 'attack hack' at the ridge. We were all hacked off 'cos we didn't hack it.