The Eleventh Doctor's nickname for the TARDIS aka Idris.
The Doctor: Sorry, do you have a name?
Idris: Seven hundred years and finally he asks.
The Doctor: But what do I call you?
Idris: I think you call me... Sexy?
The Doctor: embarassed
Only when we're alone.
Idris: We are alone.
The Doctor: Oh. Come on, then, Sexy.