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.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.