非 drew two wings spread in opposite directions — things flying apart, refusing to meet. From 'apart' came 'contrary', and from contrary came Chinese's most emphatic 'not'.
非常 'non-ordinary' is how you say 'extremely' — the most useful intensifier in Mandarin. 是非 'is and is-not' means right and wrong; a 是非之地 is a place of gossip and trouble, best avoided. Africa is 非洲, the fēi continent — pure sound-borrowing.
Two wings flying APART — things that are NOT together, NOT ordinary.
非...不可 'if not... then impossible' is a double negative that means MUST: 我非去不可 — I absolutely have to go. Chinese stacks two nots to make an unshakeable yes.