1 dialect A pimple, a spot.
2 dialect, historical A small knob situated a short distance below the brim of a metal measuring vessel to show the point of exact measure.
Late Middle English; earliest use found in Guy de Chauliac's Grande Chirurgie. Origin uncertain; perhaps compare Middle Low German forms with voiceless final consonant at plug, but these do not show an exact semantic or formal parallel. (For evidence of a possible loan in Older Scots from such Middle Low German forms see Dict. Older Sc. Tongue s.v. pluk-, ploukin(g verbal n.1 ‘the action of inserting a plug or wedge into a hole or aperture in a barrel in order to stop it up’.).