'magnesium-poor' originates from Modern English, formed by combining 'magnesium' (from New Latin 'magnesium', ultimately from Greek 'Magnesia', a place name) and 'poor' (from Old French 'povre', from Latin 'pauper', meaning 'poor' or 'needy').

'magnesium' entered English via New Latin and scientific usage (18th century) from Greek 'Magnesia' (a region); 'poor' came into English from Old French 'povre' and Latin 'pauper'. The compound 'magnesium-poor' is a Modern English formation created by joining the element name with the adjective 'poor'.

Initially, 'magnesium' referred to the mineral/element and 'poor' meant 'lacking' or 'insufficient'; when combined in Modern English the meaning has remained literal and straightforward: 'lacking magnesium'.