It's mainly used by non-Afrikaans speaking folks, as a mock of the word and the people who use i. The word poes loosely translates to "pussy/vagina" but can also mean dick. ie; he was such a poes for giving us extra work.
the proper pronounciation for poooos
would start by tightening your lips together and then forcing the 'p' sound out following that up with a 'ooh' like sound and then completing the word by hissing the 's' out.
Luke: yes, Jak...yes i do.
Carlo at McD's: I'd like one Orio-Mcfluffy to go please, poooos!!!
Sandy: Carlo don't be such a pooos.