retrieving substring with negative index, or equivalent.

Is it possible to  take a string–e.g., s1 in define([s1],[body])–and print
the substring starting from a negative index? The desired functionality is
to get the substring of a macro definition starting from the end. So
mysubstr(s1,-1) would give me `bod' dropping the y.

The most important thing I  want to be able to do is index the content of a
macro starting from the end, not from the usual beginning. Another way to
achieve this would be to create a macro that incrementally looks as the
contents of s1, pushdef()'s a new macro with the contents of s1 in
reverse–e.g., myreversestr(s1,s2) → define([s2],[ydob]). Either way, would
work.

Thank you.