POSIX_ME_HARDER is an alias for the POSIXLY_CORRECT an environment variable that can be used for some programs, particularly those written by people exasperated by some aspect of the POSIX standards. See Democracy Triumphs in Disk Units. Despite that page suggesting that df(1) uses POSIX_ME_HARDER, at least recent versions don't. So it's not a very good alias.

$ df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda3 18690576 3958640 13782496 23% / $ POSIX_ME_HARDER=yes df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda3 18690576 3958640 13782496 23% / $ POSIXLY_CORRECT=yes df Filesystem 512-blocks Used Available Use% Mounted on /dev/hda3 37381152 7917280 27564992 23% /

But it's still part of the history. :)