An idiom meaning someone who is timid or cowardly (literally 'a flea's heart').
彼はノミの心臓だから、大勢の前では意見を言えない。
He's timid, so he can't speak up in front of a crowd.
そんなに怖がるなんて、ノミの心臓だな。
You're being so scared—what a coward.
ノミの心臓のくせに、見栄を張るのはやめなさい。
Don't pretend to be brave when you're really a coward.