A cocktail containing cucumber. Several online drink recipe sites list the ingredients as:
2 oz. vodka
1 oz. dry vermouth
1 oz. lime juice
1 celery stick
1 cucumber spear
However the Screaming Viking is best known as a fictitious drink. The name was invented by Norm on an episode of Cheers ("Home is the Sailor," Season 6 Premiere, episode 122, First Aired September 24, 1987). It was used to play a trick on a know-it-all bartender named Wayne who claimed to know how to make every drink. Woody showed him up by pouring the gang some Screaming Vikings, forcing Wayne to walk off the job in disgust. This left Rebecca little choice but to hire Sam back to his old bar.
Only one ingredient is mentioned on the show: Woody asks Norm if he wants his cucumber bruised or not. Before ordering this drink at a bar, it is also worth noting that on the show everyone spits it out as soon as Rebecca isn't looking.
Sources:
http://www.webtender.com/db/drink/1594
http://www.tvtome.com/tvtome/servlet/GuidePageServlet/showid-66/epid-14194/