peak, mountaintop
岑 centers on a high, pointed landform: a peak or mountaintop. The meanings are tightly focused on the summit of a mountain.
岑 combines 山 (mountain) with 今, which likely contributes the sound. The character is used for a steep mountain peak.
The top is 山 (mountain), and the bottom 今 looks like a pointed shape. Picture a mountain with a sharp, now-visible peak: 岑 is the mountaintop.
For シン, imagine the thin, sharp peak of the mountain: thin -> シン, and the peak is so thin it looks like a needle.