From WahooPundit:

As some of the comments on your blog have alluded to: a Wahoo is a fish that can drink twice its weight. The Wahoo is a long, narrow fish — similar to a gar — with many sharp teeth and a bad temper. It certainly makes for a much fiercer mascot than a Terrapin, a Tarheel or a Hokie.
As for the official version, I defer to the Unofficial Fan Page of Virginia Sports — The Sabre.com
“Legend has it that Washington & Lee baseball fans dubbed the Virginia players Wahoos during the fiercely contested rivalry that existed between the two in-state schools in the 1890s. By 1940, Wahoos was in general use around Grounds to denote University students or events relating to them. The abbreviated Hoos sprang up later in student newspapers and has gained growing popularity in recent years.
Cavaliers is derived from those who supported the Restoration of the Monarchy during the English Civil War — in opposition to the Roundheads who backed Oliver Cromwell.