The word Unbearable is made up of three morphemes: a prefix (un), a root word (bear) and a suffix (able). Morphemes are the smallest unit of the meaning of a word but that can not stand on its own.
So, in order to infer the definition of the word, we must find out the meaning of each morpheme: un means not, bear means to support, carry the weight of or to endure and able means capable of. Unbearable, then, refers to something that can not be endured.