This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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n. The lowest sill, block, or timber supporting a building, located at or below ground level.

n. The lowest sill of a structure, usually placed in or on the ground.

n. A particularly low or dirty place/state; the nadir of something (see rock bottom)

n. A person of low status or humble provenance.

n. The lowest sill of a structure, usually embedded in the soil; the lowest timber of a house; also, that sill or timber of a bridge which is laid at the bottom of the water. See sill.

n. Fig.: A person of the lowest stratum of society; -- a term of opprobrium or contempt.

n. The lowest sill of a structure, resting on the ground.

n. A lowborn, ignorant, contemptible person.

n. Specifically, the bed-piece or bottom timber of a dam placed across the stream and usually resting on rocks or in mud.

"All of this -- but little understood within a very few years -- has been of late made generally intelligible on this side of the border, thanks, perhaps, as much to Mr. Hammond's word 'mudsill' as to any other cause."

sill