Very neat. You have tried to make a very orthogonal language from what I can see. One of the critiques of Forth is that it is postfix at runtime but typically prefix for compiling stuff. Your use of an unassigned text label being the default to which things can be attached with def is a nice solution.

How would say the language performs at runtime vs other REPL languages. Is it compiling to native code behind the curtain or is there a VM back there?