This paper considers the interplay between arbitrariness and the widely-accepted ideals of one form, one meaning and compositionality. They are shown to operate in different domains, and to clash where there is idiomaticity. Idioms provide familiar forms which are not semantically relevant to the context. In effect, this creates homonymy, which goes against any trend towards pairing one form with one meaning. The conflict can be seen as tension between two more fundamental principles. Lack of motivation is considered in an Appendix on word-manufacture, where it is shown how slippery the notion of motivation can be.