A cosmonaut, of course, is the Soviet/Russian equivalent of astronaut. The word is derived from cosmos, in its Russian meaning of space, and -naut - navigator.
The Russian word entered English at the dawn of space exploration, in the early 1960s, after Yuri Gagarin, the first cosmonaut, rocketed into orbit.
Recent protests in Russia, which began late in 2011, gave 'cosmonaut' a new meaning. It became a nickname for officers of riot police in heavy gear. Presumably, because large helmets with sliding visors resemble the look of a cosmonaut in spacesuit. It is a slang meaning mostly used by opposition bloggers and commentators on social networks.
Here's one example, from here:
На площади перед метро 'Чистые пруды' появляются омоновцы-'космонавты' в черных круглых шлемах, в нагрудниках, со щитами и дубинами.
[In the square in front of the metro station 'Chistye Prudy' there begin to appear OMON-cosmonauts in black round helmets, wearing breast-plates and carrying shields and batons.']
Read a humorous 'encyclopedic' article on the cosmonauts here with more links and examples (in Russian), and see this Tetradki article on cosmonaut and other Russian space words in English.
Photo by Александр Владимирович Плющев: 'Cosmonauts' in central Moscow.