Where do the words Sam and Will come from when it says Sam Linebacker and Will Linebacker?

Sam = Strong side linebacker
Will = Weak side linebacker

And just as a reference:

Mike = Middle linebacker

Do You Know About In A 3-4 Or 4-4 Defence What Is Used For The Two Middle Linebackers, I’ve Herd Mitch Used Before But I’m Not Sure Which One That Would Be For, Or If Mike Is Still Used?

ahem Mac Linebacker