The root word 'terminate' comes from the Latin 'terminare,' which means to limit or to end something. In contrast, the prefix 'in-' is often used in English to indicate negation or absence, as seen in words like 'incomplete' or 'invisible.' Therefore, when combined in the word 'interminable,' it conveys the idea of something that cannot be brought to an end, hence meaning endless or perpetual. This understanding of root words and prefixes is crucial for deciphering the meanings of complex words in the English language.