the Imperial signet, the great seal
壐 centers on an official seal of authority, specifically the imperial signet or great seal used to authenticate documents.
壐 combines 爾 (originally a loom or pattern, later used phonetically) with 土 (earth/clay), suggesting a seal made of clay or impressed into clay. The character is an older form related to 璽.
The top 爾 looks like a complex official pattern, and the bottom 土 is the clay that receives the imperial seal's impression.
For ジ, imagine the great seal stamped on a document with a sharp 'jee' sound: jee -> ジ, as the imperial signet presses into the clay.
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