expression
outside mutual follows; not mutuals
Twitter slang: describes a situation where you tweet at someone but neither of you follows the other. Abbreviation of 'FF外から失礼します' (excuse me for tweeting from outside FF).
FF外から失礼します。このイラスト、とても素敵ですね。
Excuse me for tweeting from outside FF. This illustration is really lovely.
FF外の人にリプライするのは少し緊張する。
I get a little nervous replying to someone who isn't a mutual.