It is the 'thousand witnesses' of theodore weld, enlarged and intensified.
1590s, alteration of well (v.) "to boil, rise;" influenced by past participle form welled. Related: Welded; welding.
plant (Resedo luteola) producing yellow dye, late 14c., from Old English *wealde, perhaps a variant of Old English wald "forest" (cf. Middle Low German walde, Middle Dutch woude). Spanish gualda, French gaude are Germanic loan-words.
"joint formed by welding," 1831, from weld (v.).