故作生人犹若外人
[gù zuò shēng rén yóu ruò wài rén]
This nickname,'Gu zu sheng ren you ruo wai ren', translates directly into acting like strangers even though we know each other. It reflects a sentiment where someone wants to appear distant to another even when there's familiarity - perhaps they've had a fallout but are trying not to show the effect.