How to say "mountain pass, frontier gap" in Arabic

A real but surprising root match: ث regularly corresponds to Hebrew ש and غ to ע (just like ثعلب/שועל 'fox'), so ث-غ-ر lines up letter-for-letter with ש-ע-ר, the root behind שער 'gate'. Don't expect a door, though — thaghr's core sense is 'a gap', literally the gap between your front teeth, extended to a mountain gap or frontier outpost (the medieval al-thughūr were border provinces). A gate is a controlled gap too, so the connection is honest, if a stretch.