a bdsm term where the Master or Mistress uses their slave as a toilet - either pissing or defecating, although pissing is more common - and generally the aim (bad choice of word) is to catch it all in the mouth with no spillage. a good toilet slave is generally a matter of pride for both concerned.
Master 1: what do you think of my new sub? she's just training to be my toilet slave.
Master 2: she's a keeper then. can i give her a go?