The phrase “cast thee by” means that the speaker would throw away or reject her child. This action might be seen as subversive in a puritanical society, where the woman’s duty was to raise children and keep the household in order. In rejecting this child, the speaker both defies her social position and takes responsibility for its existence; it is just as much her fault for casting the child away as it is for her misguided friends to steal it from her. This type of thinking reflects Puritan ideology, which held that humans are innately sinful in all actions because of Adam and Eve’s original sin.
