derisive laugh, sneer
哂 centers on a mocking or scornful laugh: a derisive sneer expressed through sound.
哂 combines 口 (mouth) with 西, which likely contributes the sound. The exact historical development of 西 in this character is uncertain.
The mouth radical 口 beside 西 suggests a mouth making a sharp, scornful sound: a sneer or derisive laugh.
For シン, imagine a sneering person making a sharp 'shin' sound through their teeth, like a hiss of contempt: shin -> シン.