It's because the kanji for Sasuke is Zuo Zhu, which is supposed to be 'to help', but 'Zhu' is phonetically similar to the 'Zhu' of 'pillar', and 'two' as in 'er' is a meme term for stupid, so he's nicknamed 'Er Zhu' which is 'Two Pillars', so basically it's a nickname to say that he's an idiot