also: はつむり
noun
iron face guard (Heian/Kamakura armor)
Historical term for a piece of armor that protected the forehead and cheeks, used during the Heian and Kamakura periods.
博物館で半首の実物を見た。
I saw an actual iron face guard at the museum.
半首は平安時代から鎌倉時代にかけて使われた防具である。
The 半首 is a piece of armor used from the Heian period through the Kamakura period.
面頬 is a broader term for face guards in Japanese armor, while 半首 specifically refers to the iron guard covering the forehead and cheeks.
Compound of 半 (half) and 首 (neck/head), referring to a partial head covering. The exact historical derivation is uncertain.