'single-grain' originates from Modern English as a compound of the words 'single' and 'grain', where 'single' meant 'one; not double' and 'grain' meant 'a seed or kernel of cereal.'

'single-grain' was formed in Modern English by combining 'single' (ultimately from Latin 'singulus' via Old French) and 'grain' (from Old French 'graine', from Latin 'granum'), producing a compound meaning 'one grain' or 'one type of grain.'

Initially, the components meant 'one' ('single') and 'seed/kernel' ('grain'); when combined their meaning has remained largely consistent as 'one grain' or 'made from one kind of grain.'