top of the head, skull
顖 refers to the top of the head or the skull, specifically the crown area. It is a rare anatomical kanji.
顖 combines 頁 (head) with 恖, which likely contributes the sound. The character is rare, and its exact historical development is uncertain.
The right side 頁 means head or page, and the left side 恖 looks like a complex shape. Imagine the top of a head (頁) with a crown-like structure (恖) sitting on it: that's the skull's top.
For シン, picture a shiny crown (シン sounds like 'shin') resting on the top of the head: shiny crown -> シン, marking the top of the skull.