Think of a sound that on its way out: low, rough, short, with no ring to it. A frog in a wet field at night makes exactly that sound, and the word is built to it. Once you can hear that , the rest of the word follows, because English uses it for any throat that produces the same thing — a raven's, or a person's when the voice has given out.
From the word’s history
Wiktionary traces croak back through Middle English to Old English crācettan ('to croak'), and says the origin is onomatopoeic — the word was made to imitate the sound.
來源把 croak 一路追到古英語 crācettan（呱呱叫），並說它是擬聲而來的。也就是說，這個字是照著聲音造出來的，所以「聲音」本來就是它的核心，不是後來引申的。
Wiktionary entry for croak (kaikki:3f6f69a190036ae4:440608360:13827): 'back-formation from Old English crācettan (to croak) ... of onomatopoeic origin.'
登入就可以看每一支怎麼連回核心畫面。