This might seem like an odd word of the week until you consider I'm a writer, LOL. One who, as it happens, is indeed brainstorming a plot that involves a kidnapping.
And yet, I actually read about this word from pure happenstance. ;-) Go figure!
Anyway. It's kinda of interesting, so let's take a look.
First of all, though sometimes moderns think kid, as applied to a child, is terrible slang that was never used in historical days, that's simply not true. The word for "a young goat" since 1200, it was extended to children in the 1500s--first written record is the 1590s, but no doubt it was used it speech before that. It was slang at first, yes, but had lost that "slang" stigma by the 1840s (though it was still considered an informal word).
So then kidnap comes to us by the 1680s--part of thieves' language. It was originally used for when they stole children to ship them to the American colonies as servants or laborers! Who knew? The kid part is therefore obvious. Nap is a variant of nab. But interesting is that kidnapper was in use at least a decade before kidnap, leading experts to believe the verb is a back-formation of the noun.
Now off I go plot out a story in which my hero kidnaps my heroine and gets way more than he bargained for, LOL.