A dancerknife was a type of energy-based weapon, a hand-held grip that generated an energy blade which appeared blue and indistinct. The blade had an adjustable speed, with a slow speed possibly being more painful.

The Orion pirates Artinton and Lasslanlin of the RRV Queen Mary wielded dancerknifes. (TOS novel: Prime Directive)

By the name and users, the dancerknife is presumably an Orion-designed weapon.