in Act I, Scene iii, line 19 the witches say "I'll drain him dry as hay" I have to explaion this quotation but i don't quite undertsand what it means. Please help!

this similie means:
this is a metaphor that simply means that whatever they are "draining" from him will be all gone. hay is dry, if you are draining something like, for instance the bathtub the water is going down the drain and leaving the tub. so drain means to take away.

oh, sorry and since hay happens to be very dry, that is what they will do. they will take it away completely.
so sorry, i don't know what it is as i haven't read the book yet :(
good luck and have fun!:)

Hay is dried silage. When it is in the field it is green and full of life. When it is cut for cattle/horse food, then it becomes brittle, yellow and verrrrrry dry....no life, no vitality.
So the witches are going to take all the vim, vigor and joy in living from Macbeth.

actually it means...well im nat trying to be a perve or anything but its that he wont able to perform sexually.