A court held thrice a year before forest verderers by the steward of the court. So called because the swans or swains were the jurymen. (Swans, swans, or sweins, freeholders; Anglo-Saxon, swan or Swein, a herdsman, shepherd, youth; our swain.)
This court was incident to a forest, as the court of pie-powder or piepoudre to a fair.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894