The spelling of the word "haddest tantrum" can be explained using the International Phonetic Alphabet (IPA). Firstly, the pronunciation of "haddest" is /ˈhædɪst/, with the stress on the first syllable. The "-dest" ending is a superlative suffix used in older English, indicating the most extreme degree of the quality described by the root word. Secondly, "tantrum" is pronounced /ˈtæntrəm/, with the stress on the second syllable. Therefore, "haddest tantrum" refers to the most severe outburst of anger or frustration someone has ever had.
The phrase haddest tantrum can be broken down into two parts: haddest and tantrum.
The term haddest is the second-person singular past tense of the verb have in Early Modern English. It means you had. Early Modern English was used during the late 15th to the late 17th centuries.
The word tantrum comes from the Latin word tantrum, meaning fit of anger or sudden outburst. It was adopted into English in the late 17th century.
So, the etymology of the phrase haddest tantrum combines the past tense form of the verb have from Early Modern English with the noun tantrum from Latin. It essentially means you had a fit of anger or you had a sudden outburst of rage.