Non-PRO Afrikaans to English Art/Literary Poetry & Literature

"It is hot in the afternoon. In the shop there are strips of biltong hanging from the ceiling, and flies everywhere. He is about to tell the boy behind the counter that he wants twenty Springbok plain when a fly flies into his mouth. He spits out in disgust.
'Sies!' says one of the other customers.
He wants to protest: 'What must I do? Must I not spit? Must I swallow the fly?' But explanations count for nothing among these merciless people."

can you please explain what is meant by "sies"?