Picture a soft cotton collar lying flat on a table. Brush starch over it, let it dry, and it stands up on its own — the same cloth, but it will not flop any more. That change is stiffen: something that used to give way now holds its shape and resists being bent. Put that change in a body and you get the two everyday uses: a person who goes still for a second when a name is mentioned, and a back that will not bend the morning after carrying boxes. Put it in something you cannot touch — a rule, a position, a person's determination — and you get the same picture again: it has stopped giving way.
From the word’s history
Wiktionary records stiffen as Middle English stifnen, that is stiff plus the ending -en. English uses that ending to build change-of-state verbs: soft becomes soften, hard becomes harden, wide becomes widen. So the word is literally make stiff or become stiff, which is exactly the collar on the table.
來源資料寫 stiffen 來自中古英語 stifnen，等於 stiff 加上字尾 -en。英文用 -en 造了一整排表示「變成／使變成」的動詞：soft→soften、hard→harden、wide→widen。所以這個字字面就是「變硬、使變硬」，正好就是桌上那片上了漿的領子。
Wiktionary entry for stiffen (packet evidence: kaikki:3f6f69a190036ae4:213729155, etymology_text: From Middle English stifnen, equivalent to stiff + -en)
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