The Collaborative International Dictionary of English v.0.48:

Tester \Tes"ter\, n. [For testern, teston, fr. F. teston, fr.
   OF. teste the head, the head of the king being impressed upon
   the coin. See Tester a covering, and cf. Testone,
   Testoon.]
   An old French silver coin, originally of the value of about
   eighteen pence, subsequently reduced to ninepence, and later
   to sixpence, sterling. Hence, in modern English slang, a
   sixpence; -- often contracted to tizzy. Called also
   teston. --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Teston \Tes"ton\, n.
   A tester; a sixpence. [Obs.]
   [1913 Webster]