skains-mates: This word, "skains-mates," appears nowhere else in the English language. "Mates" means "buddies"; "skains" may means "daggers," or "schemes." It appears that the Nurse has again gotten her words confused, but it's clear that she means that she's not a low-life.
And thou must stand by too, and suffer every knave to use me at his pleasure!: And you just have to hang around, doing nothing, and allow every wiseacre to insult me as it pleases him. However, "use me at his pleasure" could be taken to mean "have sex with me whenever he wants."