"being behind, away from the front, in a backward direction," Middle English, from back (n.) and back (adv.); often difficult to distinguish from these when the word is used in combinations. Formerly with comparative backer (c. 1400), also backermore. To be on the back burner in the figurative sense is from 1960, from the image of a cook keeping a pot there to simmer while at work on another concoction at the front of the stove.
"a cutting stroke with a weapon," 1570s, from slash (v.); sense of "slit in a garment" is from 1610s; that of "open tract in a forest" is first attested 1825, American English. As a punctuation mark in writing or printing, it is recorded from 1961.