Picture two ends that do not quite touch, and a soft metal that turns liquid before either of them does. The liquid runs into the gap, cools, and goes hard. Now there is no gap and no join you can pull apart — there is one piece. Everything the word does comes from that: the metal itself, the act of using it, and the picture writers borrow when two separate things end up holding as one.
The supplied source takes it back through Old French to Latin solidare, to make solid — the same family as solid. That is exactly what the cooled joint does: it makes two loose parts into one solid thing.
本批資料所附的來源說明：經古法文回到拉丁文 solidāre（使變結實），跟 solid 是同一家人。這正是冷卻後的接點在做的事——把兩塊鬆開的零件變成結實的一塊。
Wiktionary entry supplied in this packet: etymology_text on 'solder' (source_ref kaikki:3f6f69a190036ae4:508436342)
Two branches
真的金屬，真的接點
這一層就是畫面本身：一邊是材料，一邊是用這個材料做的動作。
noun · B2
A soft metal, often a mixture of tin and lead, that melts at a fairly low heat. You melt it onto the place where two pieces of metal or two wires meet, and when it cools and goes hard it holds them together.
焊錫
借畫面來講人和事
把接點的畫面整個搬去講抽象的事：原本分開、中間經過高溫、最後變成拉不開的一塊。這是比喻的連結，不是另一條歷史來源。
verb · C2
To join people, or the separate parts of something, so firmly that they hold as one piece — said as a picture, the way melted metal sets into a single hard joint. This use is not common; you meet it in writing rather than in speech.
把人或事緊緊結合在一起（比喻）