to destroy a great number or proportion of: The population was decimated by a plague.
2.
to select by lot and kill every tenth person of.
3.
Obsolete. to take a tenth of or from.
Origin: 1590–1600; < L decimātus, ptp. of decimāre to punish every tenth man chosen by lot, v. deriv. of decimus tenth, deriv. of decemten; see ate1
Related forms:
dec⋅i⋅ma⋅tion, noun
dec⋅i⋅ma⋅tor, noun
Usage note: The earliest English sense of decimate is “to select by lot and execute every tenth soldier of (a unit).” The extended sense “destroy a great number or proportion of” developed in the 19th century: Cholera decimated the urban population. Because the etymological sense of one-tenth remains to some extent, decimate is not ordinarily used with exact fractions or percentages: Drought has destroyed (not decimated) nearly 80 percent of the cattle.
To inflict great destruction or damage on: The fawns decimated my rose bushes.
To reduce markedly in amount: a profligate heir who decimated his trust fund.
To select by lot and kill one in every ten of.
[Latin decimāre, decimāt-, to punish every tenth person, from decimus, tenth, from decem, ten; see dek in Indo-European roots.] dec'i·ma'tion n.
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.
[des-uh-meyt] 
in Indo-European roots.]