It may be older than you think.

The Song of Solomon
5


1 I am come into my garden, my sister, my spouse:
         I have gathered my myrrh with my spice;
I have eaten my honeycomb with my honey;
I have drunk my wine with my milk.
Eat, O friends; drink,
yea, drink abundantly, O beloved.

The Distress of Separation
2 I sleep, but my heart waketh:
         it is the voice of my beloved that knocketh, saying,
Open to me, my sister, my love,
my dove, my undefiled:
for my head is filled with dew,
and my locks with the drops of the night.

3 I have put off my coat;
         how shall I put it on?
I have washed my feet;
how shall I defile them?

4 My beloved put in his hand by the hole of the door,
         and my bowels were moved for him.

5 I rose up to open to my beloved;
         and my hands dropped with myrrh,
and my fingers with sweet smelling myrrh,
upon the handles of the lock.

6 I opened to my beloved;
         but my beloved had withdrawn himself, and was gone:
my soul failed when he spake:
I sought him, but I could not find him;
I called him, but he gave me no answer.

7 The watchmen that went about the city found me,
         they smote me, they wounded me;
the keepers of the walls took away my veil from me.

8 I charge you, O daughters of Jerusalem,
         if ye find my beloved, that ye tell him,
that I am sick of love.

Note the euphemism that occurs in some of the night visiting songs of the maid opening the door to let her lover in and opening, um, herself to let him in.