心软是把刀递给别人捅自己
[xīn ruăn shì bă dāo dì jĭ bié rén tŏng zì jĭ]
It uses a metaphor: Soft-heartedness (心软) is equated to delivering a weapon to another party which could potentially harm oneself in turn. This implies a certain understanding of how kindness can sometimes place people in precarious situations, being open to being used by others.