Origin: Germanic Old English prefix
Under has 13 different meanings across 3 categories:
Being lower; being beneath something.
"The cat hid under the table to avoid the rain."
In plain English: Under means being lower than something else when you are not touching it directly.
"The under construction bridge will be closed until next year."
In a way lower or less than.
"The new employee is under two years old in terms of experience, so she will need extra training."
In plain English: Under means moving to a lower position or level than something else.
"The dog was under when he ran into the house to hide from the rain."
In or at a lower level than.
"The cat hid under the sofa to escape the rain."
The word "under" comes from the Old English under, which traces back to a Proto-Indo-European root meaning both "under" and "inside." It is related to similar words in Germanic languages like German's unter as well as Sanskrit and Latin terms for being within or below.