shouldn't we use for in these two sentences?
1) TO be drowned is used only of living thins , and means to die in water.
2) To sink is used of persons or things , and means to go down to the bottom of water.

so is it correct?
In these definitions, the word "used" has to be taken to mean "said". The expression "to be said of living things" is correct. The definition is substituting "used" for "said", which results in "to be used of living things". I myself would have chosen either "said of" or "used for", and, like you, I find it somewhat awkward to read "used of".
In my judgment it is grammatically correct, but not the best word choice. Emotion: rolleyes
thanku califjim!
Emotion: smile