To decimate something such as a group of people or animals means to destroy a very large number of them. The pollution could decimate the river's thriving population of kingfishers
To kill one man chosen by lot out of every ten in a legion or other military group
To decimate a system or organization means to reduce its size and effectiveness greatly. a recession which decimated the nation's manufacturing industry. to destroy a large part of something (decimatus, past participle of decimare , from decem )