Our transmodal capacity, or synaesthesia – i.e., the interference between our senses – commonly leads to the verbalization of a perception in terms of another, which sometimes makes it hard for us to trace back the evolution of the lexicon. For instance, many words denoting a visual perception are etymologically based on terms that originally described an acoustic sensation (e.g., Fr. éclat means both ‘violent sound and movement’ and ‘light intensity’; Germ. hell ‘bright’ originally meant ‘loud, sonorous’; Lat. clarus ‘clear, bright, gleaming’ is genetically related to calo ‘call’, clamo ‘cry out’, clango ‘to make noise’, Gr. κλέος ‘noise, glory’, κλαίω ‘cry’, κλαγγή ‘penetrating sound’, or Gr. κλάω ‘break’). We aim at drawing a detailed etymological analysis of words designating luminous phenomena mainly in the Classical and the Romance languages, with a twofold purpose: on the one hand, by identifying recurrent scenarios underlying meaning change in this conceptual area, we could shed a new light on unknown or uncertain etymologies; on the other hand, we could take a step forward towards a new understanding of how primordial linguistic expression functioned, given that linguists are starting to suggest that the origin of language itself can be thought of as a major phenomenon of synesthesia, focusing on the possibility to translate various motorial and sensorial forms into phonetic terms.