What does "I wished I had a nickel" mean when threatened by a radical extremist?

Solution 1:

The phrase "I wish I had a nickel for every time X happened" is used to mean that it happens so often that you would be rich. Other phrases are "If I had a dollar for every time X happened, I'd be rich!" It can be used with any amount of money, but using a smaller amount can indicate that the event happens so often that even a small amount builds up.

It has nothing to do with extremists themselves, and it can be used for many things. In this case, it means that being threatened by radical extremists happens often enough that the speaker could make money off it.

The phrase is also common enough that you could adapt it for conversation. For example, if you're always losing your glasses you might tell a friend:

If I had a dollar for every time I lost my glasses, I'd be able to buy a pair for every day of the year!

Letterman was making a joke, but it does seem in poor taste compared with creating a human shield.

Solution 2:

It means that, if he had (accumulated) a nickel for every time, he would now have a sizable amount of money. It's generally used as way to exaggerate, such as "if I had a penny for every time X, I'd be able to retire by now". Or, for comic effect: "if I had a nickel for every woman who fell for me, I'd be able to buy a piece of bubble gum" or "If I had a dime for every time a homeless guy asked me for change, I'd still say no."