เอาแป้งนวลไปขายชาววัง

Etymology

 * Literally, "sell face powder to palace ladies".
 * Literally, "sell face powder to palace ladies".

Verb

 * 1)  carry coals to Newcastle (do something unnecessary, redundant, or duplicative).
 * 2)  to show knowledge to someone who is more knowledgeable than oneself.