The Collaborative International Dictionary
Won't
Won't \Won't\ A colloquial contraction of woll not. Will not. See Will.
Note: Often pronounced w[u^]nt in New England.
Douglas Harper's Etymology Dictionary
won't
contraction of will not, first recorded mid-15c. as wynnot, later wonnot (1580s) before the modern form emerged 1660s. See will.
Wiktionary
won't
contraction will not; (non-gloss definition: used to indicate a future non-occurring action.)