Posko (or POSKO) is an acronym:

P: Piece
O: of
S: Shit
K: Knock
O: Off

The word was derived from a guitar in terrible shape. Scratched, terrible strings, and sounded like a dying rabbit was calling out of the amp. However, a custom made humbucker, blue steel strings, and a 5 way tone switcher fixed that. Now the word is used to describe any knock-off of popular merchandise.