Pronounced GEE-ree, this word is the
Castillian Spanish equivalent of the New World/Mexican word
gringo. It is used to refer, (generally in the
third person), to foreigners in
Spain, especially tourists.
The word is usually used neutrally, but in the right (or perhaps, wrong) tone of voice, it can be quite acerbic. There is nothing offensive about calling someone a guiri in itself, but like many tourist destinations, Spanish citizens seem to have a love-hate relationship with tourism, (a financial lifeline in many parts of the world, including Spain). On the one hand, the revenue and economic prosperity brought in by guiris is very welcome, while on the other hand, it would be nice if we would get out of the way sometimes. Or stop expecting that everyone in Spain speak English, (in the case of American guiris).
If you are traveling in Spain and don´t want the world to know that you are, in fact, a guiri, try, at a minimum, to avoid dressing like one. Avoid the guiri/gringo uniform, described excellently here. And for God´s sake, don't wear a fanny pack.