The Collaborative International Dictionary of English v.0.48:

Coistril \Cois"tril\, n. [Prob. from OF. coustillier groom or
   lad. Cf. Custrel.]
   1. An inferior groom or lad employed by an esquire to carry
      the knight's arms and other necessaries. [Written also
      coistrel.]
      [1913 Webster]

   2. A mean, paltry fellow; a coward. [Obs.] --Shak.
      [1913 Webster]