素昧平生 [- mèi pínɡ shēnɡ]have never met before; a total stranger; be a complete stranger to ...; be strangers to each other; didn't know each other before; have never had the honour of making sb.'s acquaintance; know nothing about sb.:
•¶ 我与他素昧平生。 I have never made his acquaintance.; I don't know him at all.