Where does “入口” come from?

入口 (Japanese) comes from Japanese 口, from Japanese くち, from Japanese 朽ちる.

入口 (Japanese): entrance

Definitions

入口 traces back through two separate lines of ancestry.

Step	Language	Word	Meaning
1	Japanese	口	mouth; opening; the mouth
2	Japanese	くち	mouth
3	Japanese	朽ちる	—

Step	Language	Word	Meaning
1	Japanese	入り	entering; entry, going in; entry, entering
2	Japanese	入る	to enter, go into, get into; to arrive; to join a...
3	Old Japanese	いる	—