Please explain the meaning of this quote from "Lord of the Flies." " The [pig's] half shut eyes were dim with the infinite cynicism of adult...

This happens in Chapter 8 after the hunters have killed the pig and left its head in Simon's secret place.

The idea of this quote is that grownups are cynical -- that they do not believe that anything can be ideal or good.  Cynics believe the worst of everything.  That's why the pig "tells" Simon that everything is a "bad business."

When Simon is in there, he comes to a very cynical realization.  He comes to believe that the beast is something that is inside all of the kids.  This really shows his distrust of human nature.  That's why he tells the pig that he already knows that everything is a "bad business."