Iconicity, the resemblance between the form of a word and its meaning, has effects on behaviour in both communicative symbol development and language learning experiments. These results have invited speculation about iconicity being a key feature of the origins of language, yet, the presence of iconicity in natural languages seems limited. In a diachronic study of language change, we investigated the extent to which iconicity is a stable property of vocabulary, alongside previously investigated psycholinguistic predictors of change. Analysing 784 English words with data on their historical forms, we found that stable words are higher in iconicity, longer in length, and earlier acquired during development, but that the role of frequency and grammatical category may be less important than previously suggested. Iconicity is revealed as a feature of ultra-conserved words, and potentially also as a property of vocabulary early in the history of language origins.