As Webster 1913 says below, "Juggernaut" is a version of "Jagannatha," a title of the Hindu god Krishna. Jagannatha is derived from the Sanskrit words "jagan" (world) and "nath" (lord). Each year at Puri in India, there's a procession where worshippers pull a cart carrying a giant representation of the deity, essentially a temple on wheels, and probably through accident, people have been crushed under the cart. Stories of large numbers of people throwing themselves under the cart on purpose were once widespread in Europe but are false.
Because of this ceremony, by the 19th century "juggernaut" in English came to mean either something requiring blind sacrifice, or a massive relentless force, person, institution, etc. that crushes everything in its path. (This latter "unstoppable" meaning is presumably the one the X-Men character in Mitchievious' writeup is named for.) Less metaphorically, "juggernaut" is also a somewhat pejorative British/Canadian/New Zealander name for a long articulated truck/lorry (US "tractor-trailer," 18-wheeler" or "semi"); that usage dates from the 1960s.
In 1974, Juggernaut was the name of an action movie directed by Richard Lester, and written by Richard DeKoker and
Alan Plater, featuring a terrorist who'd planted bombs aboard a luxury cruise ship and threatened to set them off unless he received his monetary demands. ("Speed meets The Poseidon Adventure," as one IMDB user put it.) Richard Harris plays the bomb expert trying to find and nullify the bombs. It also features Omar Sharif, David Hemmings, Anthony Hopkins, Shirley Knight, Ian Holm, Clifton James and Roy Kinnear. The same title was used by a 1936 Boris Karloff mystery about an evil doctor and a rich woman who plan to poison her husband; a 1968 documentary short about a truck convoy carrying the seventy-ton heart of an atomic reactor; a 2003 documentary about the Hindu ritual; a 1990 adult film; as well as the alternate title of the 1982 video game Sinistar, the subtitle of 1998's Quake 2, and the name of a 1999 Playstation game.
Juggernaut has also become the name of: a New York theater company, a New York community action non-profit, a group organizing electronic dance music events in New York City and Miami Beach, Florida, a South Carolina band who describe themselves as "Southern beer metal revivial," an English record label, a session hijacking computer program, and a remote-control truck that can be made from a kit.
Sources:
http://wordsmith.org/awad/archives/1003
http://www.pantheon.org/articles/j/jagganath.html
http://www.thefreedictionary.com/juggernaut
http://4.1911encyclopedia.org/J/JU/JUGGERNAUT.htm
http://encarta.msn.com/dictionary_1861623408/juggernaut.html
http://efl.htmlplanet.com/abc.htm
http://www.freesearch.co.uk/dictionary/juggernaut+(vehicle)
http://www.wordsmith.org/board/showflat.pl?Cat=&Board=weeklythemes&Number=115919&page=12&view=collapsed&sb=5
http://www.super70s.com/Super70s/Movies/1974/Juggernaut.asp
http://www.imdb.com/title/tt0071706/combined
http://www.imdb.com/title/tt0027824/combined
http://www.imdb.com/title/tt0237359/combined
http://www.imdb.com/title/tt0366648/combined
http://www.imdb.com/title/tt0141462/combined
http://www.imdb.com/title/tt0316616/combined
http://psx.ign.com/articles/163/163203p1.html
http://www.juggernaut-theatre.org/
http://www.juggernaut.org/
http://www.juggernaut.to/
http://www.juggernautsc.com/
http://www.oxfordmusic.info/info/labels/juggernaut.shtml
http://www.securiteam.com/tools/2NUQBSAQ0C.html