Often suggests frustration or dissatisfaction with the activity or job being given up. After endless overtime, she finally jacked in her stressful job.
Pro Tip 2/3
Sudden Decision
Implies a quick, possibly spontaneous decision to quit something. He just jacked in his studies one day and started a band.
Pro Tip 3/3
Colloquial Speech
It's conversational, fitting for informal chat rather than formal writing or speeches. I'm thinking of jacking in my gym membership – I never go.