The prescriptive infinitive can be found in the North Germanic languages, is very old, and yet is largely unnoticed and undescribed. It is used in a very limited pragmatic context of a pleasant atmosphere by adults towards very young children, or towards pets or (more rarely) adults. It has a set of syntactic properties that distinguishes it from the imperative: Negation is pre-verbal, subjects are pre-verbal, subjects are third person and are only expressed by lexical DPs, not personal pronouns. It can be found in modern child language corpora, but probably originated before ad 500. The paper is largely descriptive, but some theoretical solutions to the puzzles of this construction are proposed.