286: once, twice, thrice Sep 20, 2015

The word 'once' comes from Middle English 'ones', which is the genitive of 'one'. 'Twice' comes from the late Old English word 'twiges', from the base of 'two' + -s (in the sense of pluralization). 'Thrice' is derived of the Middle English 'thries', coming from earlier 'thrie' which in turn comes from thrīga, a word which is related to the combination of 'three' and '-s'. All three of these words were later respelled in the 16th century with a '-ce' to denote the unvoiced consonantal sound. There is no other word that would fit this pattern, so English speakers must say, "four times", rather than anything else.