This word seems to be a hapax. It would seem to be ME in form but appears only in an AN context. MED speculates that the term might be read as wateres hed.
s.
1anat.some part of female genitals, probably the hymen:
(
1321
)(the man accused of rape) prit soun vit [...] et le mit enmi le conu (l. coun) meime cete J., et li debrusa le watershad [...] et ravit sa puselage
Ed II xxvi 88
watershad (1321)