'low-calcium' is a modern English compound formed from the adjective 'low' and the noun 'calcium'. 'low' ultimately comes from Old Norse 'lágr' meaning 'not high' and entered English via Middle English, while 'calcium' comes from New/Modern Latin 'calcium', ultimately from Latin 'calx' meaning 'lime'.

'calcium' developed from Latin 'calx' ('lime') into New/Modern Latin 'calcium' (coined for the chemical substance) and then into English as 'calcium'. 'low' comes from Old Norse 'lágr' → Middle English 'low' → modern English 'low'; these elements were combined in modern English to form the descriptive compound 'low-calcium'.

Originally 'calx' referred to 'lime' (the substance); over time 'calcium' came to denote the chemical element and compounds containing it. 'low' has long meant 'not high'; combined as 'low-calcium' the phrase now specifically means 'containing little calcium'.