Think of putting your hand flat on an old stone wall and pushing. Nothing gives, nothing shifts, nothing . Solidity is the name for what your hand just found out: the thing holds, and it will still hold tomorrow.
Wiktionary gives solid + -ity, from Middle French solidite and Latin soliditas. The suffix -ity turns an adjective into the name of its quality, as in pure and purity.
Wiktionary 記載 solidity 由 solid 加 -ity 構成，經中古法文 solidité 上溯到拉丁文 soliditās。-ity 的功能是把形容詞變成「那個性質本身」，像 pure→purity。所以這個字從一開始就是在給「solid 的那個性質」取名。
Wiktionary entry for solidity, packet source_ref kaikki:3f6f69a190036ae4:1175345598
From the picture
靠得住的穩固
把手從石牆移開，改成「壓一下」一家公司、一段友誼、一份報告。推得動嗎？會不會說變就變？查得下去嗎？答案一樣是「不動」的時候，英文用的還是同一個字。
noun · C1
The quality of being steady and reliable, so that something lasts and people can depend on it — used about organisations, businesses, relationships, and about arguments or evidence that hold up when they are tested.
穩固可靠
noun · C1
How hard, firm and strong a thing is — the quality of holding its shape, not bending, breaking or giving way when weight or force is put on it.
堅固、結實