'feldspar-deficient' is a compound formed from 'feldspar' and 'deficient'. 'feldspar' originates from German, specifically the word 'Feldspat', where 'Feld' meant 'field' and 'Spat' referred to a rock/mineral. 'deficient' originates from Latin via Old French, ultimately from Latin 'deficere' meaning 'to fail or be lacking'.

'feldspar' entered English in older forms such as 'feldspath' (Middle English) and later became 'feldspar'; 'deficient' came into English from Old French/Latin forms (Latin 'deficere' → Old French → Middle English 'deficient'), and the modern compound 'feldspar-deficient' is a technical modern English formation used in geology.

Initially, the elements meant 'field-mineral' ('feldspar') and 'to fail/be lacking' ('deficient'); combined in modern usage they precisely mean 'lacking feldspar' in a rock or sample, a specialized technical sense with little semantic shift from the components.