[sbw1]

Interesting, but the connection to symbols from Japan seems a bit dubious (or at least not very recent). The term "cross out", and hence the use of an "x" to indicate negating something, seems to have been in common use in English since at least the 1920s: https://books.google.com/ngrams/graph?content=cross+out

[nardi]

I was going to say the same. Surely people have been crossing things out in writing and art for a very long time.

Edit: According to etymonline.com, crossing things out dates to at lease mid 15th century: http://www.etymonline.com/index.php?search=Cross

[renlo]

Also, it could have come from 'exit'. I've seen some text based programs use an 'x' key-press to activate a program 'exit', although I'm not sure of the chronology of its use.

[mattlondon]

This.

Everyone remembers being told at school from a very early age to "cross out" things you dont want any more (such as writing the wrong word/spelling, getting the sum wrong etc). At least in western cultures anyway, this seems fairly universal.

Seems to me to be a very easy semantic jump to go from "disregard this mistake" on paper to "disregard this thing I am looking at" on computer screens.

[gnarbarian]

A few counterpoints.

"X marks the spot"

Checking a box to indictate your selection on a form or ballot.

I can't think of any more off the top of my head.

[pjlegato]

Checkmarks (positive, accepted item) are quite often opposed to X marks (negative, undesired item).

[chippy]

Where you put a tick, you can put a cross?

[ape4]

Yeah, I could not imagine a Japanese symbol like 近く which according to Google Translate means "close" taking over.