呑舟の魚は枝流に游がず

呑舟の魚 (“boat-swallowing fish”) +‎ は (topical marker) +‎ 枝流 (“tributary”) +‎ に (locative marker) +‎ 游がず (“not swim”, continuative negative of 游ぐ (oyogu, “to swim”)).

Calque of Literary Chinese 吞舟之魚，不游枝流 from the chapter "Yang Zhu" of Liezi:

吞舟之魚，不游枝流；鴻鵠高飛，不集污池。 [Classical Chinese, trad.]

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吞舟之鱼，不游枝流；鸿鹄高飞，不集污池。 [Classical Chinese, simp.]

From: Liezi, 1^st – 5^th century CE

Tūn zhōu zhī yú, bù yóu zhīliú; hónghú gāofēi, bù jí wūchí. [Pinyin]

Boat-swallowing fish do not swim in tributaries; Giant birds fly high and do not flock to swamps.

呑(どん)舟(しゅう)の魚(うお)は枝(し)流(りゅう)に游(およ)がず • (donshū no uo wa shiryū ni oyogazu) 

1. (literal) boat-swallowing fish do not swim in tributaries
2. (figurative) larger-than-life characters do not associate with unnoteworthy characters, nor do they concern themselves with trivial matters