embryo, unfinished things
肧 centers on the early stage of development: an embryo, or by extension something still unfinished and in its formative phase.
肧 combines the flesh component ⺼ with 不, which likely contributes a phonetic element. The character is associated with the idea of an embryo or something not yet fully formed.
The flesh component ⺼ suggests something organic, and 不 can mean 'not yet'. Together, picture a tiny, not-yet-formed life growing in flesh: an embryo, something unfinished.
For ハイ, imagine a tiny embryo saying 'Hi!' from inside the womb: Hi -> ハイ, a small greeting from the unfinished life.