It means damn stupid. An amplified variant of on9
Extreme coarse language.
poison male 1: 'hey'
poison male 1: 'my sister used to work as a sales in...'
poison male 2: 'on lun 79'
on9 on stetoid. In colloquial Hong Kong Cantonese, on9 means fucking stupid, while 'on' means stupid and '9' has the same pronunciation as 'gau', meaning dick. on9, therefore, means fucking stupid (adj) or fucking idiot (n). 'on lun 7 9' mean on9 to the extreme: lun, 7, 9 all means dick in profanity in Cantonese in Hong Kong. In general, the more variations of the dick you can cram into describing the same event, in this case 'on' (stupid), the more amplified and emphasized is the meaning.
roommate1: hey what don't you turn down the volume, i wanna get some shit done for midterm tomorrow
roommate2: what don't you go bang your fucking head on the door. (you are) on lun 7 9! (adj)
Upon seeing a mainlander lining up outside the Aston Martin store for hours in the rain to get a new ride so that he can brandish in front of her fellow mainlandish friends back in the mainland, a local hk goes -
random_guy: you see that on lun 7 9 over there? who with a sane mind would line up for an Aston Marin? it hurts both the legs and the wallet. If i'm getting a AM, i'll make the sales treat me like i'm James Bond