What is the origin of omnium-gatherum?

Omnium-gatherum, “a miscellaneous collection,” has a pretty long history, considering its awkward etymology. A similar word, omnegadrium, occurs about 1430 in Middle English with the meaning “a miscellaneous collection of items in a manuscript.” Omnegadrium is a compound of the familiar Latin combining form omni– “all,” the Middle English verb gaderen “to assemble” (English gather), and the familiar Latin noun suffix –ium. Omnegadrium was re-formed to modern omnium-gatherum, which is a compound of Latin omnium “of all” (the genitive plural of omnis) and the pseudo-Latin word gatherum “a gathering,” formed from gather and the Latin noun suffix –um. Omnium-gatherum entered English in the first half of the 16th century.