'Meekrob' is a curse word that is far worse than 'shit'. It is a reference to an Thai food appetizer that is far grosser than shit, consisting of crispy noodles, poak berry and shrimp. The Knights of Standards and Practices (the Royal Order of Standards and Practices) prevent this word and other 'words of curse' from being over-used. If a curse-word is spoken too much, Gelden (the monster) will rise and destroy the Earth. Mekrob is one of the worst of these words and is hated by God, as much as it is by Eric Cartman.
(See South Park Episode #502 for more information)