Yahoo got its exclamation mark because the name already belonged to a barbecue sauce.
In 1994 two Stanford graduate students, Jerry Yang and David Filo, renamed their web guide Yahoo for its meaning, “rude, unsophisticated, uncouth.” Jonathan Swift had coined it for the brutes of Gulliver’s Travels. When Yang and Filo went to trademark it, Yahoo was already a name for knives, canoes and barbecue sauce, so they added the exclamation mark.
Yahoo on Brandseum