Wiktionary
top kill
n. (context oil industry English) a method of closing an oil well, by pumping in drilling fluid (synthetic mud) to shut off the flow of oil, and following with concrete (cement) to plug the hole
Wikipedia
Top kill
A top kill is a procedure used as a means of regaining control over an oil well that is experiencing an uncontrolled eruption of crude oil or natural gas ( blowout). The process involves pumping heavyweight drilling mud into the well. This procedure is expected to stop the flow of oil and gas from the well. A further step could be sealing the well completely, often with cement.