Green’s Dictionary of Slang

auntie n.1

also aunty
[euph.]

the lavatory.

[UK]Partridge DSUE (1984) 31/2: from ca. 1850.
[Aus]N. Keesing Lily on the Dustbin 49: ‘Going to Mary’s room’ or ‘going to visit Mary’ or ‘aunty’ are examples of a range of euphemisms used by some women.