The quick and dirty answer is that it's called The Goonies because the movie is about a bunch of goonies.

But that raises the question: what in tarnation is a Goonie?

We get a hint when we hear Mikey talk about something called "the Goondocks." We've heard of boondocks before, a term that usually refers to some type of remote area. And Astoria, Oregon is hardly a bustling metropolis.

But "goonies" itself is a totally made-up word, so don't go looking it up in Webster's. The only time the word "goonies" is used is in reference to this film. It has joined the extensive Cinema Vocab Dictionary, taking its rightful place next to words like "Wookie" and "Supercalifragilisticexpialidocious."