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jack sharp
noun
a. a low-growing plant (Emex australis) having many hard, sharp, spiny seeds that are extremely painful to step on and which will sometimes even puncture through shoe soles.
b. one of these seeds. Compare bullhead, California puncture weed, caltrop, cat head1, cat's eye1, double-gee, goat's head, three-corner jack.
Contributor's comments: I should add that, in my childhood, my first exposure to "three-corner jack" was whilst staying on a relative's farm near Virginia on the Adelaide Plains. They referred to the seeds as "jack sharps". I later foud out at school (in Adelaide) that they were usually called three-corner jacks. I was reminded of this when an older cousin used "jack-sharp" recently.