What "斗" refers to in "正斗"
The following may be folk etymology. More research is needed - specifically, textual evidence of when「正斗」started being in use among the general public.



「正斗」originated as a term in initiation rituals of Chinese secret societies or criminal underworld groups. The details of the ritual vary from source to source, but the ritual is said to involve a wooden ladle (斗).

The term「正」in「正斗」refers to the meaning orthodoxy, that is, orthodoxy of the rites or the members/leader involved in the rites of the secret society.「正斗」then refers to those who have successfully entered the society through the orthodox initiation ritual, also indicating the meaning proper, dignified. This stands in contrast to non-members or pretenders who did not know the rites (非正斗 or 假斗).

Later on, the term「正斗」permeated general society, and「正斗」gained the meaning of high quality, awesome, etc.



References:


廣東話中 -- 好正-- 一詞 - hk.answers.yahoo.com
香港黑社會用語舉隅 - mypaper.pchome.com.tw