My daughter and I were talking about rooting for a baseball team when she asked me how "root" developed to mean "cheer on." I checked the OED and found that it is U.S. slang that originated in the 1880s. Any ideas on how it developed? Is it used that way in the UK or Canada?

Surprisingly the OED says that Australian and New Zealand course slang for "root" is to "have sexual intercourse, chiefly with a man as the subject of the verb." Not sure what that means!

It's not used in the UK. I do know that the US usage causes great hilarity in the Antipodes when encountered.

Well well well! I would assume they enjoy a similar hilarity as our young people do when UK people say they were just enjoying a good fag.