'jag' originates from northern Middle English or Scots, probably attested as Middle English 'jagg(e)' or Scots 'jag', of uncertain origin; it may be related to Low German or Scandinavian dialectal words meaning 'notch' or 'point'.

'jagg(e)' (Middle English/Scots) changed in form and usage over time into the modern English 'jag', with senses extending from a physical point or notch to figurative senses such as a bout or spree.

Initially it meant 'a point, notch, or sharp projection', but over time it also came to mean 'a short bout or spree (e.g., of drinking)'; additionally the adjective form 'jagged' developed to describe things with such projections.