'cross-fertilize' is a modern English compound formed from 'cross' + 'fertilize'. 'cross' (Old English 'cros', via Old Norse/Old English usage) carried senses of 'across' and 'to combine from different sources', and 'fertilize' comes from Latin-derived 'fertile' + English verb-forming suffix '-ize'.

'fertilize' comes from Late Latin/Old French roots: Latin 'fertilis' ('fruitful') passed into Old French as 'fertile' and into Middle English as 'fertile'; the verb 'fertilize' was formed in Modern English with the suffix '-ize'. 'cross' existed in Old English as 'cros' (from Latin 'crux' in ecclesiastical contexts) and developed senses in Middle and Modern English that allowed compounds like 'cross-breed' and later 'cross-fertilize'.

Originally the element 'fertilize' referred specifically to making land or organisms fruitful; 'cross-fertilize' initially described the biological act of cross-breeding. Over time the compound gained a broader figurative sense of mixing or exchanging ideas across boundaries.