The Collaborative International Dictionary of English v.0.48:

Tester \Tes"ter\, n. [OE. testere a headpiece, helmet, OF.
   testiere, F. t[^e]ti[`e]re a head covering, fr. OF. teste the
   head, F. t[^e]te, fr. L. testa an earthen pot, the skull. See
   Test a cupel, and cf. Testi[`e]re.]
   1. A headpiece; a helmet. [Obs.]
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            The shields bright, testers, and trappures.
                                                  --Chaucer.
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   2. A flat canopy, as over a pulpit or tomb. --Oxf. Gross.
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   3. A canopy over a bed, supported by the bedposts.
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            No testers to the bed, and the saddles and
            portmanteaus heaped on me to keep off the cold.
                                                  --Walpole.
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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]

WordNet (r) 3.0 (2006):

tester
    n 1: someone who administers a test to determine your
         qualifications [syn: examiner, tester, quizzer]
    2: a flat canopy (especially one over a four-poster bed)

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "tester":
   analyst, analyzer, assayer, essayer, examinant, examiner,
   experimenter, inspector, investigator, observer, research worker,
   researcher, scrutineer, test driver, test pilot, tryer-out,
   visitator, visitor