open
叭 centers on the idea of opening, especially in the context of a mouth opening or a sound being emitted. It is a rare character whose meaning is closely tied to its visible shape.
叭 is a phono-semantic compound: 口 (mouth) suggests the meaning of opening or sound, while 八 (hachi > hatsu) provides the sound. The exact historical development is uncertain, but the component relationship is clear.
A mouth 口 that is wide open, like the shape of 八 (eight), which looks like two lines spreading apart. Together they show an open mouth.
For ハツ, imagine a mouth opening wide to say "hats!" as a sudden exclamation. The sound "hats" cues ハツ while the mouth opens.
—