If you can't hide the fact that you send a secret message, and you have no secure channel available, then resort to insecure channel, and protect your message by reshaping it so that only a key holder can extract its meaning.
Hide the method used to reshape the message (process the plaintext to its ciphertext) -- obscurity
Ideally you may want to:
Use a method that does not allow the key to be deduced from the ciphertext (equivocation)
Otherwise: use a method that would require too much time and effort to deduce the key from the ciphetext (eroding intractability)