\def\revers#1{\directlua{
   local s = "" % initial empty string
   for p, c in utf8.codes("#1") do % loop over utf8 codes in the given string
      s = utf8.char(c)..s   % insert the character before current s 
   end
   tex.print(s)  % outputs s
}}

\revers{abčde}

\bye
