'plagioclase-poor' originates from English, specifically the components 'plagioclase' + 'poor', where 'plagioclase' ultimately comes from Greek 'plagios' meaning 'oblique' and 'klasis' meaning 'breaking', and 'poor' comes from Old French 'povre' (from Latin 'pauper') meaning 'having little'.

'plagioclase' entered modern scientific English in the 19th century via German 'Plagioklas' (coined from Greek elements), while 'poor' came into Middle English from Old French 'povre' and earlier Latin 'pauper', eventually forming the compound adjective 'plagioclase-poor' in technical usage.

Originally the elements meant 'oblique' + 'breaking' for the mineral name and 'having little' for 'poor'; combined in modern usage they specifically denote 'having little plagioclase' in a rock.