A synovial cyst is a sac of fluid, which lines all tendons, muscles, ligaments, joints.
Any type of trauma or inflammation
to part of the musculoskeletal system can cause such a sac to form; very commonly, no known cause. May resolve on own, or be injected w cortisone
after aspiration, may recur. May be true that in days of yore, they were smashed with a bible, but don't recommend this approach-).