large winecup, apt, fitting
巵 centers on a large winecup, and by extension something that is apt or fitting—like a cup that is just the right size for the occasion.
巵 is a pictograph of a wine vessel. The top 厂 suggests a handle or lid, and the inner 巴 may represent the body of the cup. The exact historical development is uncertain, but the character is associated with a large drinking cup.
The top 厂 looks like a handle, and the inner 巴 resembles a rounded cup body. Picture a large ceremonial winecup with a curved handle, always fitting perfectly in your hand.
For シ, imagine a large winecup filled with a shimmering drink: the shimmering surface cues シ, and the cup is so apt for the celebration.