'single-headed' is a modern English compound formed from 'single' + 'headed'. 'single' ultimately originates from Latin 'singulus' meaning 'one each', and 'head' comes from Old English 'heafod' meaning 'head'.

'single' entered Middle English from Old French and ultimately from Latin 'singulus'; 'head' developed from Old English 'heafod' to Middle English 'hed/hedde' and then modern 'head'. The adjectival form 'headed' is formed by adding the past-participial/ adjectival suffix '-ed' to 'head'.

Originally the components meant 'one' and 'head' respectively, and when compounded they have retained the basic sense of 'having one head' or, by extension, 'having a single leader'.