From Wiktionary, the free dictionary
Sino-Vietnamese word from 頭 (“head”) and 髏, probably corrupted from Chinese 髑髏 / 髑髅 (độc lâu, “dead skull”).
đầu lâu
- (colloquial) a skull of a dead vertebrate
- sọ, hộp sọ (refers to dead or alive skulls, from a scientific point of view)