王八 originally means turtle/tortoise, long a symbol of cuckoldry in Chinese folk culture (turtles were thought to be fathered by snakes, hence 'illegitimate'). 王八蛋 — 'turtle egg' — is thus an insult about one's parentage, equivalent to 'bastard'.
register
Strong insult between adults. Softer than 操你妈 but well past polite — avoid in formal or professional settings.
