The story
Apparently from big + -ed + -y, a compound suffix used to form adjectives meaning “having the quality of”; compare uppity. Alternately, perhaps from big + -ety, a suffix used to extend monosyllabic words; compare hot diggety, hippity-hop. Attested in the Southern United States since the late nineteenth century. Commonly reanalyzed as bigot + -y.
Etymology from Wiktionary, CC BY-SA 4.0.