M.E. lafdi, lavede, ladi, from O.E. hlafdige "mistress of a household, wife of a lord," lit. "one who kneads bread," from hlaf "bread" (see loaf) + -dige "maid," related to dage "maker of dough" (see dey (1); also compare lord). "woman whose manners and sensibilities befit her for high rank in society" is from 1861
少々長い文章ですが、簡単に訳してみますと、"lady"とは、"mistress of a house hold." mistress=主婦、household=家庭ですので、「主婦」と言う意味ですね。次の "wife of a lord"の"wife"=妻、で"lord"=主人、権力者、またはキリストですので、「主人の妻」または「キリストの妻」と言う意味として使われていたようです。