late Middle English: from Latin decimat- 'taken as a tenth', from the verb decimare, from decimus 'tenth'. In Middle English the term decimation denoted the levying of a tithe, and later the tax imposed in England by Cromwell on the Royalists (1655). The verb decimate originally alluded to the Roman punishment of executing one man in ten of a mutinous legion
Historically, the meaning of the word decimate is ‘kill one in every ten of (a group of people).’ This sense has been superseded by the later, more general sense ‘kill or destroy a large percentage or part of,’ as in the virus has decimated the population. Some traditionalists argue that this and other later senses are incorrect, but it is clear that these extended senses are now part of standard English. It is sometimes also argued that decimate should refer to people and not to things or animals such as weeds or insects. It is generally agreed that decimate should not be used to mean ‘defeat utterly.’.