1) A withy was a flexible willow branch which could be used as a tie.
The OED’s single example of ‘footwithy’ is from Westmorland in 1569, and the meaning suggested is ‘a shackle for the foot of an animal’. Yorkshire provides an earlier reference: 1550 one schakill, one plewghe, one fottewethie, Killinghall and it may be that this was for the the plough foot, c.f. 1551 a ploughe foote, shakill, a tugwithe and a plowbyeme, Altofts. The ‘withy’ in such instances may by then have been made of something stronger, such as leather or iron.