In Gulliver's Travels, Jonathan Swift called humans "yahoos" and contrasted their behaviour to that of a noble race of horses that he called the Huohynym.

The yahoos have, collectively (thus the plural) many bad habits that are reminscent of the worst of humanity, including desire to engage in any behaviour for personal advantage that will disadvantage the race or species or environment as a whole.

According to its founders, the yahoo.com online service was named that specifically in order to pander to these exact desires and serve these exact people:

An individual yahoo is a person who uses the yahoo online service to pander to these desires, and presumably cannot learn critical thought - so must trust a web search engine to tell them what is important.