プログレッシブ和英中辞典(第4版)の解説
ありうべき【有り得べき】
それは有り得べきことだ
That's likely./That's not impossible.
それは有り得べからざる話だ
That doesn't sound very likely to me./That's an unlikely story!/〔皮肉に〕That's a likely story!
有り得べからざる事件が起きた
There was 「an unforgivable accident [an accident of the sort which should never have occurred].