|simpl. and trad.|
From Min Nan 我sai (wa3sai5). The colloquial reading of 我 in Min nan is wa3, and sai5 means to fuck. This phrase is popular in Taiwan from 1970s, and after the Chinese economic reform, this phrase is popular in China too. It is not offensive in mandarin.
哇塞 (traditional and simplified, Pinyin)