断口 is read as だんこう (dankou) and means fracture.
だんこう
dankou
たつ
to abstain (from)
だんねん
abandoning (hope, plans)
ことわる
to ask leave
だんてい
affirmative
だんすい
suspension of water supply
だんぜん
definitely (wrong, different, etc.)
だんじて
absolutely
だんぜつ
extinction (e.g. of a family)
だんげん
assertion
だん
decision
だんぞく
intermittence
decisive action