new moon
肭 centers on the new moon, the first thin crescent after the dark moon, marking the start of a lunar cycle.
肭 combines the flesh radical ⺼ with 内, which may contribute a phonetic element. The exact historical development linking this structure to 'new moon' is uncertain.
The flesh component ⺼ suggests something physical or organic, and 内 means inside. Picture the new moon as a thin sliver of flesh barely visible inside the dark sky.
For トツ, imagine the new moon suddenly appearing like a tots (a made-up sharp sound) in the night sky: tots -> トツ, as the crescent pops into view.