Correct spelling for the English word "hadst flash" is [hˈadst flˈaʃ], [hˈadst flˈaʃ], [h_ˈa_d_s_t f_l_ˈa_ʃ] (IPA phonetic alphabet).
There seems to be a misunderstanding or a typo in your question. The phrase "hadst flash" does not have an established etymology as it does not form a coherent term or expression. "Hadst" is a verb form in Old English, which is the past tense second-person singular form of the verb "to have". "Flash" is a separate word meaning a sudden burst of light or an instance of something happening quickly. However, these two words do not form a meaningful phrase together.