you have a stick | in your mind (AKA a pipe character)
\ lean it to back - will fall back = back slash
/ lean it to forward - will fall front = forward slash
Font features can be grouped as thickness, slant and width as in Google fonts. The natural way of writing is to lean forward which is named as slant.
Slant is one of the synonym of slash[2,3]. Here is forward slanted natural handwriting:
[2]: "slant, n.¹", Oxford English Dictionary, 1st ed., Oxford: Oxford University Press, 1911.
[3]: A slash by another name: http://www.kith.org/journals/neology/2011/12/a_slash_by_any_other_name.html