From Longman Dictionary of Contemporary Englishdeadly enemydeadly enemyENEMYsomeone who will always be your enemy and will try to harm you as much as possible The two rapidly became deadly enemies. → deadly
Examples from the Corpus
deadly enemy• Her tranquil life was threatened by the coming of a man whom she regarded as her deadly enemy.• Mr Roberts and I are deadly enemies.• The two rapidly became deadly enemies.• They're deadly enemies, those two. 