A coward or weakling; an insulting, colloquial term for someone who is easily frightened or lacks courage.
そんなことで泣くなんて、弱虫だよ。
Crying over something like that — you're such a wimp.
友達に弱虫だと言われて悔しかった。
I was upset when my friend called me a coward.
彼は高い所が苦手で、みんなに弱虫扱いされている。
He's bad with heights and everyone treats him like a coward.