noun, noun which may take the genitive case particle 'no'
daredevil; reckless; thinking nothing of one's death
Describes a person or attitude that disregards life and death; often used for someone who acts with extreme fearlessness or fatalistic abandon. Literary and somewhat formal.
He is a daredevil adventurer who thinks nothing of death.
He faced the battle with a reckless resolve, heedless of life or death.
命知らず is more common and colloquial for 'reckless' or 'fearless of death', while 死生不知 is a stiffer, literary compound.
向こう見ず emphasizes rashness and lack of foresight, whereas 死生不知 focuses on a conscious disregard for one's own mortality.
Compound of 死生 (life and death) and 不知 (not knowing, ignorance). The exact historical derivation is uncertain; the spelling is conventionally associated with the meaning of recklessness or disregard for life and death.