The name "Moscow" (or "Moskva" in Russian) is believed to come from roots meaning "dirty water" or "swampy." See: "One of the colonized territories was Moscovia. Moscow (Moskva) itself was founded in 12th century as one of the outward posts of the empire deep in the forests occupied by native Finno-Ugric tribes, the ancestors of modern Russians (80% of the topographic names in the modern central Russia have the Finno-Ugric roots including the name of the Moskva-river that means "the dirty water".

According to Rashid-ud-Din (thirteenth century) it's names comes from it's famous mosque.

According to Philotheus of Pskov (fifteenth century) it's name comes from Mosoch, the sixth son of Japhet and a grandson of Noah.

The linguists have a variety of theories, the problem being that with the -va suffix you cannot even say if it is indoeuropean or Finno-Ugric. Both use it for rivers.

If you presume it's slavic then it means swamp. There is also a Mozgava river in Poland and a Moskva river in the Ukraine. Problem is the other place names around Moscow aren't Slavic. So you get theories ascribing it just as nicely to the Sarmatians, the Finns, the Permians and the Balts.

I stick to the mosque theory because I'm no linguist.