I'm an American, so I may be wrong, but I believe it is short for "Manchester United", the most famous professional soccer team in the UK.

Lots of football teams have United in their name, so it could be any one of those teams.

It’s the abbreviation of a team name. You may find the complete name from the context.

united意思是 公司，或者团队表示名词，这儿表示具体名的团队名字省略了，因为语境中大家都知道是什么团队，例如 ，可能是 Manchester united （曼彻斯特联队)，这儿不是形容词