I didn't find an explanation or clear etymology for the kanji 関.
Kanji etymologies are often more an aspect of Chinese than Japanese. Wiktionary tends to be a good source for Chinese character origins and some sense development.
As we see there, this was originally composed as semantic (meaning) element 門 ("gate, door") + phonetic (sound) element 𢇅 (probably something like //*kroːn// in Old Chinese), referring to a door being closed or locked.
answered Feb 21 '19 at 18:39
