Here we go again!

reverse_lambda.rb

(___=->_,__=''{__=_[$.]+__; _[$.+=$$/$$]?___[_,__]:__})['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
    #
    # $. is 0 (normally, IRB has it as 1, it's ARGF.lineno)
    __ = _[$.] + __

    # Now we're incrementing it by 1 ($$ is PID, PID / PID == 1) and seeing if
    # it returns nil (false) which would be out of bounds
    #
    # If out of bounds, return our accumulator, else recurse on it
    _[$. += $$/$$] ? ___[_, __] : __
})['yllis os']