baweaver

(___=->_,__=''{__=_[$.]+__; _[$.+=$$/$$]?___[_,__]:__})['yllis os']

(___ = # So he's defining a lambda that can be self-called
  -> _, __ = '' { # with two args, one is the string and the other is an accumulator
    # We're taking the first character and putting it ahead of the accumulator and
    # setting it as the next accumulator
    #
    # $. is 0 (normally, IRB has it as 1, it's ARGF.lineno)
    __ = _[$.] + __