Stacy \s-ta-cy\ as a girl's name (also used as boy's name Stacy), is pronounced STAY-cee. It is of English origin. As a boy's name, a short form of Eustace (Greek) "good grapes; to stand". Now primarily used for girls, Stacy and Stacey are generally considered to be short forms of Anastasia (Greek) "resurrection", after the fashion of Stasia and Stasha.
For more information, see also the related name Eustacia.