@VerminusM Yiddish isn't necessarily a German dialect because it's grammar is more distinct due to the Slavic influenced Eastern dialects being the main surviving forms of Yiddish. German and Yiddish also aren't mutually intelligible. The phonology is also different (eg schön vs sheyn).
@VerminusM Yiddish is based on High German (which was a loose series of dialects spoken in Southern Germany) and encompasses Slavic loanwords in addition to Hebrew and Aramaic, especially since the Western dialects had fallen out of favor for German by Jews during the Haskalah.