Functions both as an intensifying adverb ("deadly serious") and as an adverb of manner ("strike deadly").
terriblementRenforce un adjectif ou un état ; registre courant. The room was deadly silent as the verdict was read.
La pièce était terriblement silencieuse lorsque le verdict fut lu.
mortellementAu sens propre : d’une manière capable de causer la mort. The assassin struck deadly, killing his target instantly.
L’assassin frappa mortellement, tuant sa cible sur le coup.