A “McCoy” means something that is truly genuine.
The idiomatic expression, “the real McCoy” is used when the speaker wants to emphasize the purity the authenticity of something. It is said to derive from the Scottish McKay (or MacKay) whisky whenever a host offers his guest a drop of the real McKay, whereas the idiom “real mccoy” is said to be an American variant.
Meanwhile, a decoy has quite the opposite meaning, it refers to something or someone who pretends to be something they are not. For example, a designated decoy is, supposedly, a stone-cold sober person leaving a public bar who pretends to act extremely drunk before getting into their car, and driving off. The erratic driving is aimed at drawing the police officers' attention at a sobriety checkpoint, thereby allowing the decoy's inebriated friends to drive home without being caught. (I have no idea how effective this ruse is.)
Finally, coy is just an adjective, and means; acting shy, uncertain, or unwilling to say much, often in order to increase interest in something by keeping back information about it
I was wondering whether all three terms were related or coincidental.
In addition, although the noun coyness exits, the noun coy itself doesn't seem to be listed in any of the online dictionaries I checked, is it an archaic term? If it did exist, what did the noun use to mean?