"Offtop" is not a word. It's not in any English dictionary I could find and doesn't appear in any published literature.
Matthew 7:3 "And why beholdest thou the mote that is in thy brother's eye, but considerest not the beam that is in thine own eye?"
Language evolves in mysterious ways. FWIW I find offtop to have high cromulency.
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.
[0] https://en.wiktionary.org/wiki/-%D0%B8%D0%BA#Russian