Lisp has an = function; so that binding is taken. Symbols in the keyword package are allowed to have function bindings, and so a synonym for setf can be called := (the symbol named "=" in the keyword package).

  [1]> (defmacro := (&rest args) `(setf ,@args))
  :=
  [2]> (defvar a)
  A
  [3]> (:= a 42)
  42
  [4]> a
  42
  [5]> (= a 42)
  T