'self-fertilize' is a compound of English 'self' and 'fertilize.' 'Self' comes from Old English 'self' meaning 'the same, oneself,' and 'fertilize' was formed in English from 'fertile' + '-ize' to mean 'make fertile.'

'fertilize' ultimately comes from Latin 'fertilis' meaning 'fruitful' (from the verb 'ferre' meaning 'to bear'); it entered English via Late Latin/Old French forms (e.g. 'fertiliser') and became English 'fertilize.' The compound 'self-fertilize' arose in modern English usage in biological contexts as a combination of 'self' + 'fertilize.'

Originally 'fertilize' meant 'to make (land or organisms) fruitful' and over time it became specialized in biology to mean 'to cause the fusion of gametes' or 'to transfer pollen'; 'self-fertilize' specifically denotes doing that with one's own gametes or pollen.