Decimate literally means to reduce something by a tenth, but many people use it instead to mean "totally destroy." The word "obliterate" would be a better choice than "decimate."
Decimate comes from the root "deci-" which can be found in other words involving ten, "decimal" for example.
Joe decimated the rock collection by getting rid of one tenth of the rocks.
The act of decimating someone is usually called Decimation.
"Oh yeh, you gonna decimate her?"
"Too right I am, going to be absolute decimation!"