|ˈsɒldə(r)| and |ˈsəʊdə(r)|
The word solder originates in Middle English. The Latin origin is the word solidaire, meaning to “to make solid,” which is where the -l- in solder comes from.
Solidaire (v.) – to join, be united, standing together, or interdependent.
The word more immediately comes from the Anglo-French word soudure. This Anglo French word is similar to the contemporary French verb souder. The word eventually evolved in English to its current spelling.
The modern form of the word solder in English is a re-Latinization from the early 15th century. The -l- was dropped on the way to Old French, which was common (for example, pulverem to poudre, collum to cou, calidus to chaud, etc.). Note that the -l- in solder is typically sounded in British English.
Interestingly enough, the English translation of the French word soudure refers to welding, not soldering. The word for soldering in French, soudage, is similar and shares a common root.
Souder [soo-deh] (v.) – to solder or to weld.
Avec le fil a souder, which literally means with the line of solder, applies to soldering whereas soudre a l’arc refers to arc welding. Isn’t everything a little more elegant in Latin languages? This is yet another example of a simple yet beautiful group of shared words. Why have two distinct words, weld and solder, for processes that are so similar? Almost as logical as the Standard system of measurement where lbs refers to pounds and oz refers to ounces.
Anyhow, next time you are wondering why there is a silent -l- in solder — or not silent if you live in England — you can recall this bit of knowledge and clue in your EE classmates […who will turn to you and say, “Hey, Chomsky, the Linguistics building is thataway” – Ed.].