'straight-grained' is an English compound formed from 'straight' + 'grained'. 'Straight' originates from Old English 'streht' (also spelled 'streccan' roots in Proto-Germanic), where the root meant 'not bent' or 'extended'; 'grain' originates from Old French 'graine' and Latin 'granum', where 'granum' meant 'seed' or 'grain'.

'grain' originally meant 'seed' in Latin ('granum') and later came to mean 'texture' or 'pattern' (as in wood grain) in Old French and Middle English; combined with 'straight' (Old English 'streht'), the phrase developed in Modern English to describe wood whose fibers run straight and, by extension, people who are morally 'straight' or direct.

Initially, the components referred literally to 'not bent' and 'seed/texture'; over time the compound came to mean specifically 'having straight wood fibers' and then extended metaphorically to mean 'upright' or 'straightforward' in character.