courtesy
叮 centers on courteous speech or polite verbal expression, as suggested by the mouth component 口 combined with a phonetic element.
叮 combines 口 (mouth) with 丁, which likely provides the sound. The exact historical development is uncertain, but the structure suggests a phono-semantic compound related to speech.
A mouth 口 speaking with the sound 丁 (like a polite 'ting' of a bell) represents courteous words.
For テイ, imagine a courteous person offering a cup of tea (ティー) while speaking politely: tea -> テイ.
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