Oh, it's a transliteration of Russian "офтоп", which itself started as a borrowing of "off-topic" from English (but as a noun instead of an adjective/stative) and then went some natural linguistic developments, namely loss of a hyphen and degemination, surface analysis of the trailing "-ic" as Russian suffix "-ик" [0], and its subsequent removal to obtain the supposed "original, non-derived" form.
>subsequent removal to obtain the supposed "original, non-derived" form
Also called a "back-formation". FWIF I don't think the existence of corrupted words automatically justifies more corruptions nor does the fact that it is a corruption automatically invalidate it. When language among a group evolves, everyone speaking that language is affected, which is why written language reads pretty differently looking back every 50 years or so, in both formal and informal writing. Therefore language changes should have buy-in from all users.