The earlier answer of Andreas Blass makes the meaning very clear; may I also point out that the statement carries a strong element of irony in that a person who can tolerate the more serious insult of being called a devil, often cannot bear to be given the simple information that their nose needs blowing! It is a human weakness. We are like that only. The philosopher wisely concludes that the very rare person whoever can accept both types of information, is surely worthy of your confidence, and to that person you can say anything in your mind @Sudhir Mor.
Note 2: "thou art" is simply an older English form of "you are" as pointed out by @Hot Licks in comments, and it is not specifically related to 'ta tvam asi', a Sanskrit phrase which is translated variously as Thou art that, You are that, That you are, or You're it, depending on the level of modernity and formality of the English used in the translation.
[Certain syntactic elements of Indian English and a more subtle style of inline citations are used intentionally here, so kind members please resist the urge to edit!]