The spelling of the word "Kaysi" is based on the pronunciation of the name Casey. The phonetic transcription for Kaysi is /keɪsi/, which represents the long 'a' vowel sound at the beginning of the word followed by a 's' and 'i' sound. This spelling variation is often used to give the name a unique or modern twist. However, it's important to note that variations in spelling can sometimes lead to confusion or mispronunciation.