The word "haddest sensation" is not a commonly used phrase in English. However, if we break it down phonetically using IPA transcription, we can see that "haddest" is pronounced /ˈhædɪst/ and is the archaic second-person singular form of "have". Meanwhile, "sensation" is pronounced /sɛnˈseɪʃən/ and refers to a physical or emotional feeling. Therefore, the correct spelling of the phrase would be "you have had the sensation" for modern English.
"Haddest sensation" is not a common phrase in English, so it doesn't have an established etymology. "Haddest" is the second person singular past tense of the verb "have" in Early Modern English. However, "sensation" is a different word that comes from Latin "sensatio" meaning "a sense, feeling, or perception". The combination of these words may be a modern construction or a non-standard usage.