Scholars argue whether 不 drew a bird flying up against heaven's underside or a flower's base — either way, the word for negation borrowed the shape early and permanently. The most spoken 'no' in the world's most spoken language.
不 negates almost everything: 不是 isn't, 不好 not good, 不客气 you're welcome (literally 'no politeness needed'). Tone magic: before a 4th tone, 不 flips to 2nd — 不是 búshì — a sandhi rule mouths learn before minds do.
A bird blocked by the sky's ceiling — NO passage.
对不起 'sorry' literally means 'unable to face you' — 不 buried in the middle carries the apology.