The branches of a tree with full of fruits bend towards the earth (Due to the weight of the fruits). In the same way good people are also modest ('namra') towards the other people. But the unwise people ('murkha') are like a dry stick which never bends (They do not show respect for others).
Here, subhAShitkAr comparing a wicked person with poisonous creatures. he says, scorpion's poison is in its tail, bee's poison is in its mouth. Snake has poison in its teeth. (but) a wicked person has it (poison) everywhere in his body. subhAShitkAr wants to say that these other poisonous animals are better than a wicked person because they are not "fully" poisonous as him.
The subhAshit is talking about the similarity of a coconut tree and a good person. (The coconut tree) remembers the very small amount of (salty) water which it has drunk in its early ages, and it carries very good tasty water on his head throughout its life. In the similar way good people do not forget (even a very small) favour ('upkAr') done to them.(by others)