guy
n.a man or a boy. In the plural form, people often use it to address a group of friends, regardless of their gender.
n.a man or male person. In the plural, it functions as a gender-neutral vocative to address a group of people in informal contexts.
“That guy over there is my brother.”
“I met a guy at the conference who works for the same company as you do.”
“While the term originally referred specifically to males, modern usage has shifted so that a speaker might address a room of women by saying 'you guys' without causing any confusion.”
In the plural, 'guys' is frequently used as a gender-neutral term of address in informal speech.
Learners sometimes use the plural form 'guys' when referring to a single person because they hear the plural used so frequently in greetings.
PIE word *dwóh₁ From Old French Gui, a form of Proto-Germanic Wido, a short form of names beginning with the element widuz (“wood”), such as Witold and Widukind. Cognate with Italian Guido and thus a doublet of Guido. More at wood.
From the Hebrew name גַּיְא (gáy, literally “valley”), with the spelling influenced by the existing English name.
For places which could be from either etymology above.
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.