This phrase comprises two part, but it is still following the rules which is Subject + Verb + Object.
It is a very symbolic phrase which invented in Hong Kong 70s -80s and being
popular in 90s - 2010.
On lun 99 is the emphasized form of the word on
9. They use lun and double 9 to enhance the word on9. If On 9 refer to the stupidest people, on lun 99 will then express more damage to the receiver, weakening their ability in argue. The reading
sound is from bass to treble.
sic bun chau is the second of the phase which indicate a movement. Some people directly translate it into
smd. I am not saying they are
wrong but more accurately, it is to "Suck the
soft/irr-erectional dick". Since people shall not get hard in front of an on lun 99.
Dick: Sorry sir, seems I make this report
wrong, I will fix it
asap.
Sir (smiling): Its okay, you always on lun 99
sic bun chau.
Dick: Sorry....
Sir: I will find another and you can leave now.