coping; top rail
In architecture, the protective cap or top rail on a wall, fence, or balustrade.
塀の笠木が傷んできたので、交換が必要だ。
The coping on the wall has deteriorated, so it needs to be replaced.
top part of the upper cross-beam of a torii
Specifically the uppermost horizontal beam of a Shinto torii gate.
鳥居の笠木には、しばしば反りがある。
The top beam of a torii often has a curve.
Compound of 笠 (kasa, “conical hat; umbrella-like cover”) + 木 (ki, “wood”), referring to a wooden cap or covering piece.