"Hadst notion" is a phrase that comes from Shakespeare's play "The Comedy of Errors". It refers to a mistaken or misguided belief. The spelling of "hadst" is representative of the archaic second-person singular past tense form of "have", which is no longer in common use in modern English. The IPA phonetic transcription for "hadst" is /hædst/, with the "h" sound being pronounced at the beginning, followed by the short "a" sound, a "d" sound, and a "st" sound.