mid-14c. (surname Galocher is attested from c. 1300), "kind of footwear consisting of a wooden sole fastened onto the foot with leather thongs," perhaps from Old French galoche "overshoe, galosh" (singular), 13c., from Late Latin gallicula, diminutive of gallica (solea) "a Gallic (sandal)" [Klein]. Alternative etymology [Barnhart, Hatz.-Darm.] is from Vulgar Latin *galopia, from Greek kalopodion, diminutive of kalopous "shoemaker's last," from kalon "wood" (properly "firewood") + pous "foot" (from PIE root *ped- "foot"). "The name seems to have been variously applied" [OED]. Modern meaning "rubber covering of a boot or shoe" is from 1853.
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Definitions of galosh from WordNet
a waterproof overshoe that protects shoes from water or snow;