entsetzlich
terrible
Something that causes horror, shock, or great suffering; awful, dreadful.
Die Nachrichten berichteten von einem entsetzlichen Unfall auf der Autobahn.
The news reported a terrible accident on the highway.

terrible / terribly
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terrible
Something that causes horror, shock, or great suffering; awful, dreadful.
Die Nachrichten berichteten von einem entsetzlichen Unfall auf der Autobahn.
The news reported a terrible accident on the highway.

'Entsetzlich' (terrible/terribly) is derived from the verb 'sich entsetzen' (to be horrified), which means 'sich erschrecken' (to get frightened) or 'schockiert sein' (to be shocked). Something 'Entsetzliches' (terrible) therefore causes horror.
Der Anblick der Zerstörung war entsetzlich.
The sight of the destruction was terrible.
If 'entsetzlich' comes before a noun, it means 'schrecklich' (terrible) (meaning 1). But if it comes before another adjective or adverb, it functions as an intensifier and means 'extrem' (extremely) or 'sehr' (very) (meaning 2).
Das war eine entsetzliche Vorstellung. (Bedeutung 1)
That was a terrible performance. (Meaning 1)
In the meaning 'schrecklich' (terrible), 'entsetzlich' (terrible) is a very strong word. It expresses deep shock, horror, or disgust and is used for truly bad events or conditions.
Die Lebensbedingungen im Lager waren entsetzlich.
The living conditions in the camp were terrible.
As an intensifier, 'entsetzlich' (terribly) is often combined with negative feelings or states. Typical combinations are: 'entsetzlich müde' (terribly tired), 'entsetzlich langweilig' (terribly boring), 'entsetzlich kalt' (terribly cold), or 'entsetzlich teuer' (terribly expensive).
Mir ist heute entsetzlich kalt.
I'm terribly cold today.
very
'sehr' and 'entsetzlich' can both be used for emphasis. 'sehr' is neutral and works in many positive, negative, and factual situations. As an intensifier, 'entsetzlich' means more like 'extremely' and sounds much stronger. It is especially suitable for negative feelings or unpleasant states, for example pain, fear, cold, or tiredness. The words can be confused because both increase intensity. But 'entsetzlich' is not neutral: it makes the sentence more dramatic and usually more negative. For normal emphasis, 'sehr' is safer.

Am Abend war Hannes entsetzlich müde und wollte nur noch schlafen.
In the evening, Hannes was terribly tired and only wanted to sleep.
Am Montag ist sie sehr konzentriert in den Unterricht gegangen.
On Monday, she went to class very focused.