laugh, ridicule
嗤 centers on expressing scorn through laughter: laughing at someone or something in a mocking, ridiculing way.
嗤 combines 口 (mouth) with 蚩, which likely contributes the sound and may be related to a worm or insect, but the exact historical development is uncertain.
A mouth 口 beside the insect-like 蚩 suggests the buzzing, irritating sound of mocking laughter, like a swarm of tiny jeers.
For シ, imagine a person sneering 'sheesh' under their breath: sheesh -> シ, and the dismissive sound matches the ridiculing laugh.
sneer