Noun. Late 15th century.
1 A pretended friend; an unavowed enemy. obsolete exc. dial. L15
2 A supporter, a backer. L16
I love the ambiguity of this word; a backfriend can either be a backstabbing traitor in waiting or a faithful companion that will never leave you behind. You've noted, of course, that the negative meaning of backfriend is now considered obsolete, but then that's exactly what a conniving backfriend would want you to think, right? Whichever it is, the seeds of doubt have been planted: you'll never again hear the words 'I got your back, bro,' without feeling a gnawing sense of unease.