Adverb: Below, lower, beneath
Adjective: Lower in position or degree; on the underside
Verb Phrase: Go under
Prefix: Signals subordination, situation beneath something else, of a lesser extent, or insufficiency
The origin of under is Old English under, which comes from Proto-Germanic *under- from Proto-Indo-European *ndhero- ("lower"). The Old English word could carry the sense of "subordination" (like underling), as well as be used as a preposition meaning "between, among" (as in under these circumstances). It was also used as a prefix as it is today.