means A person who joins pieces of metal together by melting and fusing them, typically using intense heat from an electric arc or a gas flame.

from From "weld," a 16th-century alteration of the older verb "well," meaning to boil up, bubble, or gush — the same root behind the "well" you draw water from. The idea was of metal heated until it surged and flowed together. The intrusive "d" likely crept in from the past participle "welled," and the spelling stuck. Add the agent suffix "-er," and you have the one who makes metal well up and merge.