apologies for tweeting to someone you don't mutually follow
Twitter jargon; abbreviation of ＦＦ外から失礼します. Used when replying or tweeting at someone who doesn't follow you back, acknowledging the breach of netiquette.
FF外失します. Sorry for the sudden reply out of the blue.
FF外失ですが, just a word on that topic.
Half-width variant; less common but still seen.
Abbreviation of the Twitter phrase ＦＦ外から失礼します (FF外から失礼します), where ＦＦ stands for 'follow-follow' (mutual following) and 外 means 'outside'. The exact origin is uncertain but it emerged as internet slang on Japanese Twitter.