The term flail refers to two different weapons: a two-handed infantry weapon derived from an agricultural tool and a one-handed weapon. The defining characteristic of both is that they involve a separate striking head attached to a handle by a flexible rope, strap, or chain. The two-handed variant saw use in a limited number of conflicts during the European Middle Ages.
It's the main weapon of Kinu the Canary. Her weapon is an unusual variation of the one-handed flail (commonly known as nunchaku) that is connected by a retractile chain and has sharpened blades in one of the extremes. Her stealth kill animations suggest the chain is sharpened as well. Kinu uses this weapon exclusively with one hand.
In Gameplay Edit
In gameplay this weapon makes Kinu capable of attacking enemies from a distance and easier to pull and stagger guarding enemies, but also give her a delay between attacks and force her to assume combat position to make attack combos. Her stealth kills are also slower than Goh's and tend to throw enemy bodies a little distance away, the particular exception is killing downed/fainted enemies which is much quicker and certain.
See Combat for Kinu's moveset.