expression, noun
timidity; cowardice; courage of a flea
Idiomatic expression literally meaning 'a flea's heart', used to describe someone who is very timid or cowardly.
彼はノミの心臓だから、高いところには絶対に行かない。
He's so timid he would never go to high places.
ノミの心臓の私には、ホラー映画は無理だ。
With my cowardly nature, horror movies are impossible for me.
Literally 'a flea's heart', an idiomatic expression comparing a person's courage to the tiny heart of a flea, implying extreme timidity.