As used here, it means someone that lacks the technical training to perform a certain service but takes the job anyway, hoping to learn at the customer's expense. In most derogatory form it describes someone that doesn't have technical training, will never get that training, knows they're likely to scrwew things up, and hopes they can BS the customer enough to get paid.
As an example, there was a post asking how to clean Oriental rugs. The rugs were large and challenging and the advice here was to take them to a rug cleaning plant but the cleaner did them in his garage anyway, choosing whatever advice seemed to sound good. After cleaning, he posted questions about how to remove dyebleed, didn't remove the dyebleed, delivered the rugs to the elderly customer and bragged that he got away with it because her eyesight was bad.
That's a hack.