na-adjective
unfortunate but expected; harsh but fair
Internet slang abbreviation of 残念だが当然. Used when a disappointing outcome was predictable or logically inevitable, often with a resigned or wry tone.
あのチームが負けるのは残当だよ。
It's unfortunate but expected that that team lost.
彼が遅刻するのは残当だな。
It's harsh but fair that he's late.