erübrigen

Etymology
From, hence “to leave over, have as surplus”. Accordingly, the reflexive actually means “to be left, remain” and one would originally say  in the sense of “XY still remains to be done”. The quite opposite contemporary sense seems to emerge around 1900. It probably developed through confusion with the active verb in the sense of “to spare”. Moreover, that which is left to be done last is often the least important thing and may in fact become redundant through developments in the meantime.

Verb

 * 1)  to leave over, have extra, put aside, spare
 * 2)  to be or become gratuitous, redundant, no longer necessary
 * 3)  to be left, to remain to be done
 * 1)  to be or become gratuitous, redundant, no longer necessary
 * 2)  to be left, to remain to be done
 * 1)  to be left, to remain to be done