class Thread

me[:oliver] = "a" #=> RuntimeError: can't modify frozen thread locals
me.thread_variable_set(:oliver, "a") #=> RuntimeError: can't modify frozen thread locals
me.freeze
me = Thread.current

Thread#thread_variable_set, nor can fiber local variables be set.
Freezes the thread so that thread local variables cannot be set via

def freeze
  _locals.freeze
  super
end