久假不歸

Originally, “to borrow the name of renyi (benevolence and righteousness) without actually practicing them”. From Jin Xin I of Mencius:

孟子曰：「堯舜，性之也；湯武，身之也；五霸，假之也。久假而不歸，惡知其非有也。」 [Classical Chinese, trad.]

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孟子曰：“尧舜，性之也；汤武，身之也；五霸，假之也。久假而不归，恶知其非有也。” [Classical Chinese, simp.]

From: Mencius, c. 4^th century BCE, translated based on James Legge's version

Mèngzǐ yuē: “Yáo Shùn, xìng zhī yě; Tāng Wǔ, shēn zhī yě; wǔ bà, jiǎ zhī yě. Jiǔ jiǎ ér bù guī, wū zhī qí fēi yǒu yě.” [Pinyin]

Mencius said, 'Benevolence and righteousness were natural to Yao and Shun. Tang and Wu made them their own. The five chiefs of the princes feigned them. Having borrowed them long and not returned them, how could it be known they did not own them?'

久假不歸

1. to borrow something and not return it for a long time

By interpreting the characters as 久假不歸 (jiǔ jià bù guī), the idiom could be mistaken to mean "to be on vacation for a long time and not return".