The spelling of "not let out peeped" is based on its pronunciation. The word "not" is spelled as /nɒt/, with the "o" pronounced as a short vowel sound. "Let" is spelled as /lɛt/ with the "e" pronounced as a short vowel sound. "Out" is spelled as /aʊt/, with the "ou" pronounced as a diphthong. "Peeped" is spelled as /pipt/, with the "ee" pronounced as a long vowel sound and the "ed" as a final consonant. Overall, the spelling of this phrase reflects its phonetic composition.