What is won't a contraction of?

Isn’t comes from is not. Doesn’t comes from does not. What the heck does won’t come from?

Will not. Because it’s hard to say “willn’t”.

Would not, at a guess.

Per http://www.wisegeek.com/what-is-a-contraction.htm:

The contraction for would not is wouldn’t.

L and W sounds are not as far away from each other as the alphabet would have you believe. Think of Polish ł (pronounced w), or the way French turns one cheval (shev-al) into many chevaux (shev-o). Neither is an explanation for won’t, already given above; just to show that the change from -illn- into -on- isn’t quite as bizarre, linguistically, as it seems at first glance.

It’s a contraction of the Middle English wol not - Chambers English Dictionary.

http://www.onelook.com/?w=won%27t&ls=a says it is a contraction of woll not. Woll is then defined in older dictionaries as will.

Kids often say words like “yewwow” or “wowipop” for this reason.

I stand corrected and shamed.