OINK-FILLED PEN is a term used to describe a writing instrument that is filled with the sound of a PIG. The term is composed of two words: STY (3 letters) and PIGSTY (6 letters). STY is an old English word meaning 'enclosure for animals', while PIGSTY is a compound word derived from the word 'PIG' and 'STY'. Together, these two words form the term OINK-FILLED PEN, which is used to describe a writing instrument that is filled with the sound of a PIG.