隔壁老王 (gébì Lǎo Wáng, 'Old Wang next door') is a famous Chinese internet meme referring to a suspiciously friendly male neighbor. It's used humorously to imply an affair - similar to the Western 'mailman' joke.
usage
隔壁 is more specific than 邻居 (línjū, neighbor). 隔壁 means literally 'separated by a wall' - the immediately adjacent unit. 对面 (duìmiàn) = across the way. 楼上/楼下 = upstairs/downstairs.
