To become blue or bluish in color, especially due to lack of oxygen or cyanosis.
From cyano- (blue) + -ose (a verb-forming suffix). Related to cyanosis, the medical condition of bluish discoloration.
When someone's lips or fingernails cyanose, it's literally your body waving a red flag (ironically in blue) that it's not getting enough oxygen—it's one of the most obvious visual signs doctors look for in emergencies.
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