To join two pieces of metal together by heating and fusing them, or to unite things together firmly.
From Old English 'wieldan' (to wield, control, or meld), related to 'well' meaning to boil. The word shifted from general uniting to the specific metalworking technique.
Welding wasn't reliable until electric welding was invented in 1890—before that, blacksmiths had to heat metal white-hot and hammer it, which is why bridges and ships are so much stronger now with modern welded seams.
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