A jag is a sharp area that protrudes from a surface, a pointed bump or knob. The walls of a cave, rather than being smooth, might be lined with rocky jags.

You'll encounter jags when you're mountain climbing or walking along the rocky coast. To "be on a jag" or "go on a jag" means to be completely unrestrained, whether you're on a drinking jag or a crying jag. This second meaning is of U.S. origin, first meaning "a load of hay or wood," and later "as much drink as a man can hold," in the 17th century. By the 1880s, it was "a period of unrestrained activity."

Definitions of jag
  1. noun
    a sharp projection on an edge or surface
    “he clutched a jag of the rock”
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    type of:
    any solid convex shape that juts out from something
  2. noun
    a flap along the edge of a garment; used in medieval clothing
    synonyms: dag
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    type of:
    any broad, thin, and limber covering attached at one edge; hangs loose or projects freely
  3. noun
    a slit in a garment that exposes material of a different color underneath; used in Renaissance clothing
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    type of:
    a long narrow opening
  4. verb
    cut teeth into; make a jagged cutting edge
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    make saw-toothed or jag the edge of
    type of:
    separate with or as if with an instrument
  5. noun
    a bout of drinking or drug taking
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    type of:
    decadence, intemperance, intemperateness, self-indulgence
    excess in action and immoderate indulgence of bodily appetites, especially in passion or indulgence
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