CoffeScript was designed with transcompilation to JavaScript in mind. This is why you've correctly noted they're conceptually similar in many respects, and therefore relatively easy to transcompile.
Transcompilation can be relatively simple in the case of CS to JS, or significantly more involved in the case of Ruby to Java. One of the most significant examples of transcompilation is PHP to C++, as in the case of Facebook's HipHop compiler.
Yeah, I still don't agree with you. If I were compiling to JVM bytecode, would you no longer consider it a transcompiler? And if that bytecode were exactly what javac produced after compiling the Java source I emit now...what exactly is the difference?
FWIW, I'm probably going to just emit JVM bytecode anyway, since it's easier than emitting syntactically correct Java source.
This is where the lines get blurry. Anything that translates source code to bytecode would clearly not be a transcompiler as bytecode can hardly be considered source code, even if an abstract implementation of the machine ends up executing it.
Something is considered source code if it is intended to be written and read primarily by humans. Once upon a time, machine code would have been the same as source code as there were no compilers, and punchcards were punched with opcodes and data directly. Now, nobody in their right mind considers machine language to be source code, and I could only make a very tenuous argument in favor of JVM bytecode being source code.
Compilation is the process of producing something the "machine" understands natively. Traditionally this was the CPU. Your CPU doesn't understand Ruby, but it (probably) understands x86 machine code, along with various extensions. In the case of Java, the machine is no longer the CPU, but an abstraction of it. The machine in this case only understands JVM bytecode, not Java. Same with the CLR which only understands IL, not C#, F#, VB.NET, etc.
If you emit bytecode directly you'd have a compiler not a transcompiler.
Transcompilation can be relatively simple in the case of CS to JS, or significantly more involved in the case of Ruby to Java. One of the most significant examples of transcompilation is PHP to C++, as in the case of Facebook's HipHop compiler.