unerträglich
unbearable
So unpleasant, painful, or disturbing that one can hardly or not at all endure it.
Die Hitze in der Dachgeschosswohnung war im August unerträglich.
The heat in the attic apartment was unbearable in August.

unbearable
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unbearable
So unpleasant, painful, or disturbing that one can hardly or not at all endure it.
Die Hitze in der Dachgeschosswohnung war im August unerträglich.
The heat in the attic apartment was unbearable in August.

The word is built logically: the prefix 'un-' negates the verb 'ertragen' (to endure), and the ending '-lich' turns it into an adjective. So it literally means 'not to be endured'.
Der Lärm von der Baustelle ist einfach unerträglich.
The noise from the construction site is simply unbearable.
To express for whom something is unbearable, you use the preposition 'für' followed by the accusative. This construction is very common.
Für empfindliche Ohren ist diese Musik unerträglich.
For sensitive ears, this music is unbearable.
'Unerträglich' (unbearable) often describes strong sensory perceptions or feelings. Common combinations are 'unerträgliche Hitze' (unbearable heat), 'unerträglicher Schmerz' (unbearable pain), 'unerträglicher Lärm' (unbearable noise), or 'unerträglicher Gestank' (unbearable stench).
Nach der Zahn-OP hatte er unerträgliche Schmerzen.
After the dental surgery, he had unbearable pain.
impossible
'unmöglich' and 'unerträglich' can both describe something or someone very negatively. For people or behavior, 'unmöglich' mainly means that something is completely rude, inappropriate, or unacceptable. 'unerträglich' puts more emphasis on the fact that you can hardly stand something, for example noise, pain, or a very annoying person. When talking about bad behavior, 'unmöglich' often sounds more natural. When talking about heavy strain or something very disturbing, 'unerträglich' is more precise.

Der Lärm im Haus ist so unerträglich, dass ich mich nicht konzentrieren kann.
The noise in the house is so unbearable that I can't concentrate.
Sein Verhalten beim Gespräch war unmöglich.
His behavior during the conversation was unacceptable.