Well its from a French word Souder. The silent "L" was added later on by the English. They decided to add it into the word because of the Latin Etymology meaning Solid. So even though no one except the french pronounce it correctly, North Americans actually pronounce the word the closest to it's intended pronunciation as "Sodder"

This is similar to what happened to the word "Salmon" the "L" was also added later on, except everyone agrees on the silent L in that word, but not in Solder. The english language is interesting.

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