Where does “顎が落ちる” come from?

顎が落ちる (Japanese) comes from Japanese が, from Finnish -nka — Forms nouns, mainly bird names (in dialects) and some toponyms.

Ancestry of “顎が落ちる”, step by step

顎が落ちる traces back through 3 separate lines of ancestry.

via Japanese が

StepLanguageWordMeaning
1JapaneseがIndicates the subject of a sentence; conjunction;...
2Finnish-nkaForms nouns, mainly bird names (in dialects) and some toponyms

via Japanese 落ちる

StepLanguageWordMeaning
1Japanese落ちるto fall; to go down; to trip
2Old Japanese落つto fall

via Japanese 顎

StepLanguageWordMeaning
1Japanese顎jaw; chin; barb
Every word from Finnish -nka →Every word from Japanese が →