Yes the real phrase is "Don't mistake kindness for weakness". Do not assume that because someone is nice/kind, he/she is also weak/stupid can be taken advantage of! :)

It means that you shouldn't assume someone that is being kind can easily be taken advantage of.

i think that kind of kindness is actually strength because being kind with those kinds of people takes a lot of effort and patience! customer service is a hard skill, but i've mastered it by saying "take it to the corporation. thanks!" .. lol. hate it when customers get ticked off saying things like "THAT is a mondo burrito?!?! looks like a taquito to me." well take that mondo burrito and shove it up ur *** dont bullshit me about it.. i think i've said more than my share, lol. but yup, kindness in the worst situations is true strength!