sip
嚌 centers on taking in a small amount of liquid: sipping, tasting, or drinking delicately.
嚌 combines 口 (mouth) with 齊, which likely contributes the sound and may suggest a measured or even action, fitting the idea of controlled sipping.
The mouth 口 beside 齊, a shape associated with evenness or alignment, suggests taking in liquid in a controlled, even manner: sipping.
For セイ, imagine sipping a drink and saying 'say' after tasting it: say -> セイ, as you sip and pronounce your approval.