guy

Using the word 'Guy' to refer to a male human comes from Guy Fawkes (1570-1606), who was the most famous of a group of men who plotted to blow up the English parliament in 1605, as revenge for persecution of the Roman Catholics. The gunpowder plot failed, and Guy was hanged.

November 5 is still known as Guy Fawkes Day (or Bonfire Night), and an effigy called a guy is built by children to be burned that evening. Because of this, 'guy' later came to mean a foolish person (much in the same way as had 'dummy'), and, by natural progression, later to be an informal term for any male person.

Nowadays 'guys' can refer to a mixed group of males and females, or sometime a group of only females. A girl alone will almost never be referred to as a guy, although the traditional 'gal' is falling out of use, and I predict that it is only a matter of time before guy is a unisex term.

Guy (?), n. [Sp. guia guide, a guy or small rope used on board of ships to keep weighty things in their places; of Teutonic origin, and the same word as E. guide. See Guide, and cf. Gye.]

A rope, chain, or rod attached to anything to steady it; as: a rope to steady or guide an object which is being hoisted or lowered; a rope which holds in place the end of a boom, spar, or yard in a ship; a chain or wire rope connecting a suspension bridge with the land on either side to prevent lateral swaying; a rod or rope attached to the top of a structure, as of a derrick, and extending obliquely to the ground, where it is fastened.

© Webster 1913.

Guy, v. t. [imp. & p. p. Guyed (?); p. pr. & vb. n. Guying.]

To steady or guide with a guy.

© Webster 1913.

Guy, n.

1.

A grotesque effigy, like that of Guy Fawkes, dressed up in England on the fifth of November, the day of the Gunpowder Plot.

The lady . . . who dresses like a guy. W. S. Gilbert.

2.

A person of queer looks or dress.

Dickens.

© Webster 1913.

Guy, v. t.

To fool; to baffle; to make (a person) an object of ridicule.

© Webster 1913.