23333 (er san san san san) - A Chinese equivalent of American LOL and the Japanese "www", especially as users wanted to convey their amusement more quickly. It may sound a bit like "ah-hahahaha", and more 3's can be added to emphasize one's amused reaction. In video games, it instead means "retreat, retreat!" or "spread out!"
哈哈哈 (ha ha ha) - Its meaning is obvious from its pronunciation.
哲♂學 (zhe♂xue) - Philosophy♂. Used more often in China and Taiwan as a tag or a term for Gachimuchi-related things.
鬼畜 (gui chu) - Kichiku, a term borrowed from Japanese that originally referred to monster characters (such as the majority of Touhou girls) that later refers to MAD music remixes like those from Nico Nico Douga.