The phrase "hadst fling" is an archaic form of English meaning "you had thrown." The spelling is reflective of the pronunciation at the time it was in common use, which can be represented in IPA phonetic transcription as /hædst flɪŋ/. This uses the archaic second person singular form "hadst" for "had" and the simple past tense form "fling" for "thrown." While this form may be unfamiliar to modern speakers of English, its use can add a touch of antiquity and poetry to writing.