You can think of upstanding as one posture: something has come up onto its feet and is holding there. A post set straight in the ground. A whole room rising as an important person walks in. English then lends that same posture to a person's character, so that someone who stands straight is someone who does not bend when bending would be easier.
From the word’s history
The packet's Wiktionary extract records two entries: the honest adjective is from up- + standing, and the standing-up adjective is from the verb upstand. Both are built from the same two pieces, so the picture of standing straight sits under each of them.
來源的 Wiktionary 抄錄分成兩個詞源條目：「正直」這個形容詞來自 up- 加 standing，「站立」這個形容詞來自動詞 upstand。兩邊用的零件一樣，所以「站直」這個畫面在兩個意思底下都成立。這是詞源的紀錄，不是我們替它編出來的故事。
Wiktionary extract supplied in packet (kaikki:3f6f69a190036ae4:366959176 'From up- + standing.'; kaikki:3f6f69a190036ae4:366962002 'From upstand.')
登入就可以看每一支怎麼連回核心畫面。