Why is yahoo.com called Yahoo!?
We'll let the people at Yahoo! explain where the name came from.
The Web site started out as "Jerry and David's Guide to the World Wide Web" but eventually received a new moniker with the help of a dictionary. The name Yahoo! is an acronym for "Yet Another Hierarchical Officious Oracle," but Filo and Yang insist they selected the name because they liked the general definition of a yahoo: "rude, unsophisticated, uncouth." Yahoo! itself first resided on Yang's student workstation, "Akebono," while the software was lodged on Filo's computer, "Konishiki" - both named after legendary sumo wrestlers.
—The Fact Monster