倒刺 refers to two distinct but related things: a hangnail (the small, painful piece of torn skin at the side of a fingernail) and a barb (the backward-facing point on a fishhook or arrowhead that prevents easy removal). The literal meaning is 'reverse thorn' — the 'reverse' direction is what makes it catch or hurt. For the hangnail sense, it's a common everyday word; for the barb sense, it's more technical (fishing, archery, or engineering).