Best known from the proverb 吃一堑，长一智 - each setback you fall into teaches you something. The literal sense is a defensive moat or a natural chasm acting as a barrier (天堑).
Sits on top and supplies the sound (zhǎn shifting to qiàn). It faintly suits the meaning too: a moat is a deep cut sliced into the ground, echoing the 'chop, sever' sense it carries elsewhere.
