From Esperanto ŝelo (“skin”).
selo
- The outside, surface, outer layer or boundary (e.g. skin, bark, peel, shell).
- (by extension) Shape, form.
2014 May 25, Sonja Lang, “Preface”, in Toki Pona: The Language of Good, page 7:mi wile pana e selo pini pi toki pona.- I hope to present the language [Toki Pona] in its completed form.