'50-50' originates from modern English, specifically the compound word 'fifty-fifty', where 'fifty' originally meant 'five tens' (from Old English 'fīftig').

'50-50' changed from the spelled form 'fifty-fifty' (a compound of two instances of 'fifty') and eventually came to be written in numeric form as '50-50' in modern usage. The element 'fifty' itself evolved from Old English 'fīftig' meaning 'five tens'.

Initially it referred literally to two equal parts of fifty each; over time it evolved into the idiomatic sense of 'an equal share' or 'equal likelihood' (an even chance).