'multi-grain' is a compound formed from the prefix 'multi-' (from Latin 'multus', meaning 'many') and 'grain' (from Old French 'graine', from Latin 'granum', meaning 'seed' or 'grain').

'multi-' entered English via Latin (and Old French) as a productive prefix meaning 'many'; 'grain' comes from Old French 'graine' < Latin 'granum'. The compound 'multi-grain' is a modern English formation, becoming common in the 20th century in food labeling and marketing.

Originally, 'grain' referred to a seed or cereal seed and 'multi-' simply meant 'many'; over time the compound came to specifically mean 'containing several types of cereal grains', especially in the context of bread and cereals.