Unbearable means you cannot put up with it. Heat that keeps the whole street awake, a noise from the flat above that starts at eleven, a silence at the table that goes on too long.
Take it apart and it is un- plus bear plus -able: not able to be borne. The bear inside is not the animal but the old sense of carrying, the one in I cannot bear it and bear the cost. The adverb is unbearably, and it loves a hot afternoon: unbearably hot, unbearably slow, unbearably smug. People are called unbearable too, which means insufferable rather than in pain. The opposite, bearable, is a low bar — bearable means only just survivable, never good.
1adjectiveunbearableToo painful, unpleasant or annoying to put up with.
“The pain in his jaw became unbearable by the evening.”
synonyms: intolerable, insufferable
2adjectiveimpossibleImpossible to spend time with.
“He is unbearable for a week whenever his team loses.”
Unbearable is home-made. Middle English already had unberable, un- and bearable pushed together. The bear at its heart is the old verb meaning to carry, the same one in bear a load and bear the cost. What is unbearable is what you cannot carry, so you put it down.
Middle English
unberable
English, today
unbearable
From Middle English unberable, equivalent to un + bearable.
Etymology from Wiktionary, CC BY-SA