[lwh]

The delete/rubout key on many old terminals had an X on it. Like this: http://www.cosam.org/images/vt220/keyboard.jpg

[boobsbr]

The backspace key on my Lenovo keyboard is identical. And if I remember correctly, it was the same key on an electrical typewriter I used in the 90s.

[DanBC]

On typewriters you would backspace and then overtype an X. Those keyboards tend to just use the word "backspace" or a back arrow symbol.

So typewriters use this symbol when they automate the overstrike or they moved to correction tape.

[TeMPOraL]

So that's where they took that Backspace icon on smartphone keyboards from...