One way to hold `stalwart`: something built to stay standing. Picture a thick gatepost sunk deep in the ground, or a wide-shouldered person planted in a doorway — you can push, and it does not move. English then carries that picture over from bodies to positions. A stalwart supporter is the one still standing where they stood after everybody else moved away, and a stalwart of the club is the person who has been holding the place up for years.
From the word’s history
The packet traces the word to Middle English stal-worth and Old English stǣlwierþe, glossed as able to stand in good stead, serviceable, probably from staþol (foundation) plus -wierþe (able to, capable of). So standing firm and can be relied on were in the word from the start.
字源資料把它追到中古英文 stal-worth 和古英文 stǣlwierþe，意思大約是「站得住、派得上用場」。前半可能來自 staþol（地基、立足之處），後半 -wierþe 是「能夠、值得」。所以「穩」和「靠得住」從一開始就在這個字裡面，今天的用法只是把同一個畫面從身體移到立場。
Wiktionary, "stalwart"
登入就可以看每一支怎麼連回核心畫面。