Pinyin initialism of
楼主 (lóuzhǔ, 'building owner' = thread starter). Forum threads in Chinese are pictured as buildings (
楼): the opening post is
一楼 'first floor', each reply is a
楼层 ('floor', i.e. comment level), and the OP is the
楼主. LZ is the textspeak version, popular on Tieba, Zhihu and Weibo when typing the characters is too slow.
