butting in; off-topic reply; interjecting in a thread
Internet slang for replying to a discussion thread or message board conversation with an unrelated or unsolicited comment, or the reply itself. Often used when someone jumps into a conversation without invitation.
Please stop making off-topic replies that have nothing to do with the thread.
The conversation got derailed by his interjection.
Compound of 横 (yoko, 'side, horizontal') and レス (resu, from English 'response'), literally 'side response'. The term originated in Japanese online message boards and mailing lists.