To understand

https://www.etymonline.com/word/understand

Extract

Old English understandan “comprehend, grasp the idea of,” probably literally “stand in the midst of,” from under + standan “to stand” (see stytand (v.)). If this is the meaning, the under is not the usual word meaning “beneath,” but from Old English under, from PIE *nter- “between, among” (source also of Sanskrit antar “among, between,” Latin inter “between, among,” Greek entera “intestines;” see inter-). Related: Understood; understanding.

That is the suggestion in Barnhart, but other sources regard the “among, between, before, in the presence of” sense of Old English prefix and preposition under as other meanings of the same word. “Among” seems to be the sense in many Old English compounds that resemble understand, such as underniman“to receive,” undersecan “examine, investigate, scrutinize” (literally “underseek”), underðencan“consider, change one’s mind,” underginnan “to begin.” It also seems to be the sense still in expressions such as under such circumstances.