A scribe who wanted to cancel a passage drew a cross of two diagonal strokes over it. The letter shaped like a cross of two diagonal strokes is kher, and the Russian language took the letter name and made a verb of it meaning to cross something out, to strike it, to have done with it. It is one of a very small number of cases anywhere in which a letter name becomes an ordinary verb of the language.
Two things are worth drawing out of it. The first is that letter names, in an alphabet whose names are words, do not stay inside the alphabet. They leak into speech, into idiom and into slang, and the tradition of naming letters after meaningful words guarantees this in a way that the modern practice of naming them after their sounds does not.
The second is the practice of correction itself. Slavonic manuscripts show a settled repertoire for it: a line struck through, dots placed above or beneath a word to mark it for deletion, a cross drawn over a passage. There is no erasure in the modern sense, because scraping parchment is expensive and visible, so a correction is normally a mark rather than a removal, and the cancelled text stays legible underneath. That is a considerable gift to editors. A corrected Slavonic manuscript preserves the reading its scribe rejected as well as the reading he preferred, and much of what is known about how the translations were revised comes from exactly such marks.