Think of a prince walking into a room. Everything around him is on a bigger scale than ordinary life — the house he lives in, the welcome he gets, the money in his hand. Princely can point at the prince himself (whose family, whose land, whose title), or at anything done on that scale. And because the scale is set so high, the word is easy to turn into a joke: put it in front of a very small number and the gap does the work.
The supplied Wiktionary entry records princely as Middle English princely / pryncely, that is prince plus the adjective ending -ly, so the word literally says 'prince-like'. Every later use is still doing that: either belonging to a prince, or made to a prince's standard.
packet 裡的 Wiktionary 條目記錄 princely 來自中古英語的 princely／pryncely，也就是 prince 加上形容詞字尾 -ly，字面上就是「像王子那樣的」。今天不管是講歸屬還是講規格，其實都還在做同一件事。
Wiktionary entry 'princely' (adjective), source_ref kaikki:3f6f69a190036ae4:1026696743, supplied in this packet
From the picture
王子等級的排場與金額
把「屬於王子的」轉成「做到王子那一級」：說話的人不是在講歸屬，而是在借王子當作一把尺，量眼前這件事的規格。
adjective · C1
Grand, impressive, or very generous, in the way something belonging to a prince would be. It is used about buildings, gifts, welcomes and other things done on a large and expensive scale.
氣派的；豐厚大方的
adjective · C1
Belonging to a prince, or connected with a prince — his family, his title, or the land he rules.
王子的；王侯的