<unk>  refers to a finger which is bent in a particular way:  bent toward the palm at joint nearest the knuckles, and bent back away from the palm at the joint furthest from the knuckles (hyperflexion at the proximal interphalangeal joint with hyperextension at the distal interphalangeal joint.
Causes are injury, inflammatory conditions like rheumatoid arthritis.
Boutonniere deformity
Haglund deformity
Nursemaid's elbow
Swan neck deformity
Rocker bottom foot
Golfer's elbow
Cenani Lenz syndactylism
Freckle