outside a gate; beyond one's expertise
Literally 'outside the gate'; figuratively, refers to something outside one's field of knowledge or experience. Often used in the phrase 門外漢 (もんがいかん) meaning 'layman' or 'outsider'.
門外で待っていてください。
Please wait outside the gate.
その話題
That topic is outside my area of expertise.
門下 means someone who belongs to a teacher's school, while 門外 means someone outside it or unrelated to the field, so the two are near-opposites despite sharing 門.
outsider; layman; amateur
never taken out of the premises; kept strictly within the family/group; not for public viewing
outside one's area of expertise; outside one's field
Compound of 門 (gate) and 外 (outside). The figurative sense is a natural extension of the literal meaning.