Found in the following dictionaries
SOWPODS (Collins Scrabble Words)
"Groundsill" Definition
noun
The horizontal timber nearest the ground in the frame of a building.
Source: American Heritage Dictionary
A timber beam used as the foundation for a building.
Source: Wiktionary
The lowest beam of a door-frame; the threshold.
Source: Wiktionary
The timber of a building which lies next to the ground; the ground-plate; the sill.
Source: Century Dictionary
In mining, the bottom piece of a wooden gallery-frame.
Source: Century Dictionary
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