Example sentences
fēi非cháng常gǎn感xiè谢。
Very grateful.
zhè这fēi非cháng常hǎo好。
This is very good.
About this word
非 is not, 常 is constant or ordinary, so 非常 is literally "not ordinary". That literal reading goes back to classical Chinese; its use as a plain degree adverb, the everyday equal of very, is a later development that is now fully standard. Two usage points. First, 非常 is stronger than 很 and leans toward writing rather than casual talk. Second, since both are degree adverbs they cannot stack: 很好 and 非常好 are both fine, but 很非常 is impossible in any register.
The story behind the character
非
非 is two wings. In the oracle bone it is a pair of wings drawn back to back, pulled apart and pointing away from each other. Xu Shen read it the same way: 非, 违也, taking the wings from 飞 and using their turned-away look to mean going against. The word for wrong and for not began as two wings refusing to fly together.
常
常 is 巾, cloth, with 尚 giving the sound. Xu Shen wrote 常, 下帬也: a lower garment, a skirt. That was the first meaning, and the character 裳 is its descendant. Constant, ordinary and often all arrived later by borrowing, which is why 非常 can mean both extremely and unusual. It starts with a skirt.