According to the Han-dynasty dictionary Shuowen Jiezi, the oracle-bone 不 pictured a bird flying up into the sky and simply not coming back down — wings spread, tail trailing beneath a line for the heavens. Other scholars instead see the cupped base of a flower, the part left behind once the petals fall away. Either way, the picture was borrowed purely for its sound, and only the meaning "not, no" survived.
A piece of culture
The character that changes its own tone 不 is one of only two characters in Mandarin (the other is 一) whose tone shifts depending on what follows: alone it is bù, but before another fourth-tone syllable it flips to bú, as in 不是 (bú shì, "is not"). On the oracle bones, 不 was already indispensable — carved constantly, because after nearly every royal divination the diviner had to record a yes or a no. It paired often with 雨: 不雨, "it will not rain," the flip side of the very question you met at 雨.