The Spanish word 'tranquillo' meaning 'small step' has an interesting journey from ancient Celtic languages. It starts with the Gaulish word 'tarinca', which meant 'iron bar'. This evolved into the Spanish word 'tranca' meaning 'bar' or 'pole'. From 'tranca', we get 'tranco' meaning 'step' or 'threshold'. Finally, 'tranquillo' was formed by adding the Spanish diminutive suffix '-illo' to 'tranco', literally meaning 'small step'.

This evolution shows how a word for a physical object (an iron bar) gradually came to refer to a step or threshold, possibly because bars or poles were used as thresholds or to create steps in ancient times. The addition of the diminutive suffix makes the word refer specifically to a small step.

Some simpler Spanish words that share this etymology are 'tranca' (bar, pole) and 'tranco' (step, threshold). These words are more commonly used and might be familiar to Spanish learners. While 'tranca' maintains the original meaning of a bar or pole, 'tranco' shifted to mean a step or threshold, and 'tranquillo' further specifies the size of such a step.

While there aren't direct English cognates from this Gaulish root, it's worth noting that this word is distinct from the similar-looking Spanish word 'tranquilo' (meaning 'calm' or 'quiet'), which comes from Latin 'tranquillus' and is related to English words like 'tranquil' and 'tranquility'.