'high-tax' originates from Modern English as a compound of 'high' and 'tax'. 'High' ultimately comes from Old English 'heah' meaning 'tall' or 'great', and 'tax' comes from Medieval Latin 'taxare' (via Old French 'taxer') meaning 'to estimate, assess.'

'tax' changed from Medieval Latin 'taxare' into Old French 'taxer' and Middle English 'taxen', eventually becoming the modern English word 'tax'. 'high' comes from Old English 'heah' and developed into Middle English 'high' before becoming modern 'high'.

Initially, the elements meant 'tall/great' ('high') and 'to assess/estimate' ('tax'); the compound has come to mean 'subject to relatively large taxation' in modern usage.