Think of pressing your hand down on something. It does not sink, it does not bend, it does not move, and it is the same however long you press. Nothing you do changes its shape. Both meanings of the word grow out of that one moment of pushing and getting nothing back: a floor or a piece of ground that will not give, and a person whose position will not give either.
From the word’s history
The packet's Wiktionary entry gives the word as un- + yielding. 'Yield' is to give way, so the word is literally 'not giving way', and both meanings are that same phrase read two ways: a thing that does not give way under weight, and a person who does not give way under pressure.
評估資料裡的 Wiktionary 記的是 un- 加 yielding。yield 本來就是「讓開、屈服」，所以這個字字面上就是「不讓」。兩個意思等於同一句話的兩種讀法：東西被壓不讓，人被施壓也不讓。
Wiktionary etymology supplied in this evidence packet (source_ref kaikki:3f6f69a190036ae4:1138313186), text: 'From un- + yielding.'
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