The story
A late 18th century construction, either a classical compound with components derived from Ancient Greek μᾰ́ντῐς (mắntĭs, “mantis, prophet or seer”) + -λογία (-logía, “-logy, branch of study, to speak”), otherwise syncretically from either: the Greek element μᾰ́ντῐς (mắntĭs) + English elements -o- + -logy or Greek element μᾰ́ντῐς (mắntĭs) + English element -ology.
Etymology from Wiktionary, CC BY-SA 4.0.