Full Idea
Unnatural, when it means anything, means unfrequent.
Gist of Idea
Unnatural, when it means anything, means infrequent
Source
Jeremy Bentham (Intro to Principles of Morals and Legislation [1789], II.14 n8.9)
Book Reference
Mill,John Stuart: 'Utilitarianism (including On Liberty etc)', ed/tr. Warnock,Mary [Fontana 1962], p.52