From Tuscon Weekly (more at the link):
English derives from the Germanic dialects of the Angles, Saxons and Jutes who conquered southern Britain in the 5th and 6th centuries A.D. When they arrived, the native Britons spoke Celtic. Latin was also common among upper crust Brits, having adopted it during four centuries of Roman occupation. Surprisingly, though, the Germanic invaders absorbed hardly any Celtic, with the exception of place names. (Such as London, from a Celtic word meaning “wild.” Britain itself is a rendering of the native term for Wales, which was later applied to the entire island). And while English eventually borrowed loads of Latin, that mostly came a millenium later.
As the Anglo-Saxon upstarts settled in, English began to develop as its own language, distinct from the Germanic dialects that begat it. But then along came the Vikings, another Germanic people speaking what we now call Old Norse. Viking raids escalated into full scale invasions until by the 9th century they had conquered parts of the British Isles, including Northern Scotland, and were on the verge of overrunning all of England. But in 871, Saxon King Alfred the Great rallied the locals to defeat them at Ashdown, stemming the tide. By treaty, the Saxons and Vikings established a boundary between them that became known as the danelaw.
The English language thus survived the Viking onslaught. However, commerce across the danelaw boundary infused English with loads of words from Old Norse. A few examples include the pronouns they, them and their, as are skin, sky, smile, and wrong, along with law. Tellingly, husband is also from Old Norse, whereas wife is from Old English, thus reflecting male marauders marrying local women.I love this stuff. And who knew that tomorrow is "Mother Tongue Day"? Thank you, United Nations!