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 by Cromwell on the Royalists (1655)
Historically, the meaning of the word decimate is ‘kill one in every ten of (a group of people)’. This sense has been more or less totally superseded by the later, more general sense ‘kill, destroy, or remove a large proportion of’, as in the virus has decimated the population. Some traditionalists argue that this is incorrect, but it is clear that it is now part of standard English.