From Wiktionary, the free dictionary
The particle よ (yo2) expresses exclamation, while the particle し (si) strengthens it.
大魚よし (opuwo yo2si) (kana おふをよし)
- oh great fish, allusion to しび (sibi1, “tuna”)
711–712, Kojiki, poem 110:意布袁余志斯毘都久阿麻余斯賀阿禮婆宇良胡本斯祁牟斯毘都久志毘- opuwo yo2si sibi1 tuku ama yo2 si ga areba ura ko1posike1mu sibi1 tuku Sibi1
- (please add an English translation of this quotation)