A SEARCHING INTO

The word investigation was borrowed into the English language in the middle of the fifteenth century from the Old French word investigacion, which is a fourteenth century loanword from Latin investigationem, meaning "a searching into". That word is composed out of the prefix in-, which here meant "into", the verb vestigere, or "to search", and the suffix -onem, which forms an accusative singular noun. In- eventually traces to the Proto-Indo-European root en, which had the same definition, and vestigere comes from the Latin word for "footprint", vestigium, which has an uncertain origin but might trace to a Proto-Indo-European reconstruction sounding something like steyg and meaning "to walk". Investigate is a sixteenth-century back-fornation; usage of both that and investigation has been trending upwards in recent decades.