One of the mysteries of the English language finally explained.
In England: (the title of) an officer of state having custody of the Great Seal; = "(Lord) Keeper of the Great Seal".
Late 16th century. Shortened from post-classical Latin custos sigilli regis keeper of the king's seal from classical Latin custos + sigillī, genitive of sigillum + rēgis, genitive of rēx.
custos sigilli/ˌkʌstɒs sɪˈɡɪli//ˌkʌstɒs sɪˈdʒɪlʌɪ/
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