From 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]