「そんなに舐められた相手じゃないけどなぁ・・・」「そうだよ、窮鼠猫を噛むって言うじゃないか」

「・・・」「、」

"They're not someone to be underestimated, though..." "That's right — you know what they say: a cornered rat will bite a cat."

Literal

"That-much looked-down-on opponent [copula-じゃない] [though-けどなぁ]..." "That's-right, cornered-rat cat [object-を] bites [quotative-って] say [don't-they-じゃないか]."

A dialogue built around the proverb 窮鼠猫を噛む (きゅうそねこをかむ, 'a cornered rat bites a cat'). 舐められた ('was looked down on / was underestimated') is the passive of 舐める in its figurative sense. The first speaker hedges with けどなぁ (trailing doubt). The second speaker reinforces with the proverb, using って言うじゃないか ('don't they say...?') — a common way to invoke a saying as support. The proverb means that even the weak will fight back when cornered.