是非
ぜひ · zehi · JLPT N4 · adverb
without fail
是非 = right + wrong: right or wrong, do it either way
是 means right and 非 means wrong. Together they originally meant right and wrong, pros and cons. As an adverb the sense became: whatever the pros and cons, regardless — so, by all means, without fail.
是 — sun 日 + 正 (correct/upright)
Originally a compound suggesting 'the sun 日 directly overhead'—straight, correct, true. From this concrete sense of 'right' or 'this very thing,' 是 came to mean 'this' as a demonstrative, and later the modern copula 'to be / is.' The 'correct' meaning survives in words like 是非 (right and wrong).
非 — two opposing wings 羽-like form
Originally a pictograph showing two wings spread in opposite directions, symbolizing things facing away from each other. From this 'opposition' image came the meaning 'not' or 'wrong.' It is also used as a phonetic component in many characters like 排, 悲, and 罪.