I have been misinterpeting this word for a long time, it seems, having believed it meant "to reduce to a tenth of former size or amount". However, Mary Anne has it right (any surprise?).

decimate

TRANSITIVE VERB: Inflected forms: dec·i·mat·ed, dec·i·mat·ing, dec·i·mates
1. To destroy or kill a large part of (a group). 2. Usage Problem a. To inflict great destruction or damage on: The fawns decimated my rose bushes. b. To reduce markedly in amount: a profligate heir who decimated his trust fund. 3. To select by lot and kill one in every ten of.

ETYMOLOGY: Latin decimre, decimt-, to punish every tenth person, from decimus, tenth, from decem, ten. See dek in Appendix I.

OTHER FORMS: deci·mation —NOUN

USAGE NOTE: Decimate originally referred to the killing of every tenth person, a punishment used in the Roman army for mutinous legions. Today this meaning is commonly extended to include the killing of any large proportion of a group. Sixty-six percent of the Usage Panel accepts this extension in the sentence The Jewish population of Germany was decimated by the war, even though it is common knowledge that the number of Jews killed was much greater than a tenth of the original population. However, when the meaning is further extended to include large-scale destruction other than killing, as in The supply of fresh produce was decimated by the nuclear accident at Chernobyl, only 26 percent of the Panel accepts the usage.

-- The American Heritage® Dictionary of the English Language: Fourth Edition. 2000.