Definition of niggling for English Language Learners. : causing you to feel a slight pain or to be worried or annoyed for a long time.
What is PPAP? The Production Part Approval Process (PPAP) is a standardized process in the automotive and aerospace industries that helps manufacturers and suppliers communicate and approve production designs and processes before, during, and after manufacture.
1 Paracetamol (or acetaminophen), an analgesic drug, also known as APAP from active ingredient acetyl-para-aminophenol.  Automatic Positive Airway Pressure, a medical device used to treat breathing disorders like sleep apnea.