A nailgun is a power tool that drives nails into wood (and sometimes other materials) in one fast push, so the worker does not have to swing a hammer again and again. You hold it against the wood and pull the trigger, and the nail goes in instantly. Most nailguns on building sites run on compressed air from a hose and a compressor, which is why people describe them as air-powered, but there are also electric ones, battery ones, and gas ones. Carpenters, roofers and builders use them because they are far faster than a hammer over a whole day's work. The word is written nailgun or nail gun; both spellings are common. It is a plain, practical word — the language of workshops and building sites rather than of books. It is countable: a nailgun, two nailguns. Because it is fast and strong, it is also a genuinely dangerous tool, and you will often meet the word in safety warnings and in news stories about accidents.