pump knot 
In Appalachian speech, a "pump knot" is a raised bump or swelling, usually from getting hit hard on the head.
Hillbilly Dude Says...
Pronunciation
[PUMP-not]
Meaning & Usage
- Raised bump from an injury (noun)
Elmer:
An' I reckon somebody'll leave here with a pump knot on their head.
Estel:
Hope it ain’t me.
other spellings: pump-knot, pumpknot, and goose egg
★ If you hear someone threaten a "pump knot," they probably mean business - and you might want to step back. ★
Origin
Likely comes from the resemblance of a swelling to the rounded knobs on an old-fashioned water pump handle. The term "knot" for a lump or bump dates back centuries in English.
Notes
"Pump knot" is most often used in playful or exaggerated threats, but it can describe any noticeable swelling on the head. "Goose egg" is a similar term used elsewhere.