languages which can overwrite code like Lisp can of course overload symbols, and is well known to be one of the most flexible languages around. Postscript also allows for redefinition of symbols freely. But the king of brevity in terms of languages is perennially FORTH, whose programmers take delight in rewriting the meaning of + temporarily, and the results are incredibly impressive in both speed and compactness. The problem with FORTH is that it is basically a write-only language, and it is super hard to understand your own code a month later, much less other people's code, because you have to keep in your mind what is on the stack, and how many operators are consumed by each operand. Another language which is highly flexible is Red/Rebol.
In my efforts to develop my Beads language i have moved in the complete opposite direction so that any feature which makes things harder to read is removed, and the whole thrust is to make programs minimal while promoting clarity. For example + in JS means add OR could mean string concat; i use + for add, and & for concat so you can't mix them by mistake (or cause ruinous implied type conversions which JS does so readily).