Old English writan "to score, outline, draw the figure of," later "to set down in writing" (class I strong verb; past tense wrat, past participle writen), from Proto-Germanic *writanan "tear, scratch" (cf. Old Frisian writa "to write," Old Saxon writan "to tear, scratch, write," Old Norse rita "write, scratch, outline," Old High German rizan "to write, scratch, tear," German reißen "to tear, pull, tug, sketch, draw, design"), outside connections doubtful. Words for "write" in most I.E languages originally mean "carve, scratch, cut" (cf. Latin scribere, Greek grapho, Sanskrit rikh-); a few originally meant "paint" (cf. Gothic meljan, Old Church Slavonic pisati, and most of the modern Slavic cognates).
To write (something) off (1680s) originally was from accounting; figurative sense is recorded from 1889. Write-in "unlisted candidate" is recorded from 1932.