noun
not wearing a helmet; riding without a helmet
Commonly used in the context of motorcycles, bicycles, or construction sites. Often implies a violation of safety rules.
from English no helmet
See also: ノーヘル
ノーヘルメットでバイクに乗ると罰金です。
Riding a motorcycle without a helmet will get you a fine.
彼はいつもノーヘルメット
He always rides his bicycle without a helmet.