From Discworld & Terry Pratchett Wiki
A human constable in the Watch. He is mentioned by Vimes in The Truth when complaining about the new speaking tubes installed in the watch HQ. Apparently they had crossed the speaking tubes with the pneumatic message tubes, resulting in Keenside having all his teeth knocked out by an unexpected message capsule. Vimes is assured that this can't happen again (because Keenside doesn't have any more teeth).