This work focuses on a pattern of tonal alternation that is intertwined with a pattern of reduplication in Nigerian Pidgin. In the language, verbs are reduplicated to iconically express iteration. To convey that the iterated event occurs in an irregular or dispersive manner, the verb bears a low tone (L) on all its tone-bearing units (TBU), while the reduplicant bears a high tone (H) on all its TBUs. The resulting L-H tonal melody is considered the exponent of an irregular marker, while the intertwined reduplication is considered the exponent of an iteration marker. Due to the similarity between the exponent of the irregular marker and the iconic tonal melody of ideophones that express the semantic notions of irregularity across languages, the form-meaning mapping of the irregular marker is regarded as a grammaticalised form of the tone melody in the substrate ideophones. This suggests that ideophones can contribute to the emergence and expansion of grammar, as well as the typology of grammatical tone. Considering that perceptual resemblance between linguistic structures and the structural components of real-world elements is the basis of iconicity, the pattern of tonal alternation in Nigerian Pidgin suggests that the notion of perceptual motivation in linguistic theory is not purely phonetic and phonological but also includes the crossmodal perception of sensory imagery.