It was vaguely familiar, and the Master joined in, humming a few of the words, joining in halfway on a few lines. The height difference was enough to make it easier to wrap an arm around the Doctor's waist than his shoulder in this state, but that was fine, cozy and fine, and if they were wobbling a little as they headed to their room, it wasn't enough to knock them over as long as they leaned on each other.

Probably some sort of metaphor in that.