The Spanish word 'indecoroso' (meaning 'indecorous' or 'improper') comes from the Latin word 'indecorosus', which meant 'dishonorable'. This Latin word was formed by combining three elements: the prefix 'in-' meaning 'not' or 'without', the root 'decorum' meaning 'honor' or 'respect', and the suffix '-osus' meaning 'full of'. When these elements came together, they created a word that literally meant 'full of not having honor' or 'lacking proper respect'.

The word maintained its meaning as it evolved from Latin to Spanish, continuing to describe behavior or things that go against proper conduct or social decorum.

Some related Spanish words that might be familiar to Spanish learners include 'decoroso' (decent, proper) - which is the opposite of 'indecoroso' as it lacks the negative 'in-' prefix. There's also 'decoro' (decorum, propriety), which comes directly from the Latin 'decorum' and represents the core meaning of proper behavior or dignity.

English speakers might recognize the connection to several similar words including 'decorous' (showing proper behavior), 'indecorous' (not proper or appropriate), and 'decorum' (proper behavior). These words share the same Latin roots as the Spanish 'indecoroso'. The English word 'decoration' is also distantly related, as it stems from the same idea of what is fitting or proper, though it evolved to refer more specifically to ornamental additions that are appropriate or becoming to something.