When you can't tolerate something, it's unendurable. The vintage camper your parents bought might smell so terrible that it's unendurable.
Use the adjective unendurable to describe situations that are truly impossible to bear, like the unendurable pain of losing a beloved friend. You can also use the word to emphasize the negative qualities of something: "This math class is unendurable — I'm transferring to modern dance." When you can endure something, you can stand it, even if it's hard. Endure comes from the Latin indurare, "make hard" or "harden the heart against."