early 14c., soudur, from Old French soldure, soudeure, from souder, originally solder, "to consolidate, close, fasten together, join with solder" (13c.), from Latin solidare "to make solid," from solidus "solid" (see solid (adj.)).
Modern form in English is a re-Latinization from early 15c. The loss of Latin -l- in that position on the way to Old French is regular, as poudre from pulverem, cou from collum, chaud from calidus. The -l- typically is sounded in British English but not in American, according to OED, but Fowler wrote that solder without the "l" was "The only pronunciation I have ever heard, except from the half-educated to whom spelling is a final court of appeal ..." and was baffled by the OED's statement that it was American. Related: Soldered; soldering. The noun is first attested late 14c.