expression
apologies for tweeting at someone you don't follow and who doesn't follow you
Slang abbreviation used on Twitter when initiating a conversation with someone outside one's mutual follow circle. The full phrase is ＦＦ外から失礼します.
Ｆ外失ですが、先ほどのツイートについて質問があります。
Sorry for tweeting at you out of the blue, but I have a question about your earlier tweet.