expression
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。突然のリプライ失礼します。
FF外失します. Sorry for the sudden reply out of the blue.
FF外失ですが、その話題について一言。
FF外失ですが, just a word on that topic.
The full, more formal version of the same phrase; used in the same context but less abbreviated.
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.