The story
PIE word *méǵh₂s From mega- (prefix meaning ‘very large, great’) + -lith (suffix meaning ‘stone’) + -ic (suffix meaning ‘of or pertaining to’, forming adjectives from nouns), probably modelled after monolithic; according to the Oxford English Dictionary the word is first attested earlier than megalith. It is analysable as megalith + -ic.
Etymology from Wiktionary, CC BY-SA 4.0.