'parallel-grained' is a compound formed from the adjective 'parallel' and the past-participial adjective-forming element from 'grain' (past participle 'grained'), meaning 'having grain.' 'Parallel' originates from Greek via Latin and French, and 'grain' originates from Old French and Latin.

'parallel' comes from French 'parallèle', from Late Latin 'parallēlus', from Greek 'parallēlos' (para- 'beside' + allēlos 'one another'). 'Grain' comes from Old French 'graine', from Latin 'granum' meaning 'seed' (later extended to mean 'small particle' or 'texture'). The compound 'parallel-grained' is a descriptive compound formed in Modern English to denote the arrangement of grain.

Individually, 'parallel' originally conveyed the idea of 'side by side' and 'grain' originally meant 'seed' or 'small particle'; combined in Modern English they evolved into the current descriptive meaning 'having grain that runs parallel,' applied especially to wood and materials.