Meaning

verb kill one in every ten, as of mutineers in Roman armies

verb kill in large numbers

originThe verb is first attested in 1591, the noun in 1641; borrowed from Latin decimātus, perfect passive participle of decimō (“to kill one tenth; to tithe”) (see, from -ate (verb-forming suffix) and -ate (noun-forming suffix)), from decimus (“tenth”) + -ō (verb-forming suffix). As a noun, via Latin decimatus (“tithing area; tithing rights”).

Etymology from Wiktionary, CC BY-SA 3.0