c. 1600, contraction of will I, nill I, or will he, nill he, or will ye, nill ye, literally "with or without the will of the person concerned." See nill + will (v.1).
willing
will-o'-the-wisp
willow
willowy
willpower
willy-nilly
Wilsonian
wilt
wily
Wimbledon
wimp