Thunder joined english in the 1200s, the old English word was ‘þunor’ (þ being pronounced the same as the modern ‘th’), it came to English from the proto-germanic word ‘thunraz’ which sounds silly but meant the same thing. ‘Thunraz’ was also the origin of the Norse word for thunder ‘þorr’ from which the Norse god/marvel superhero ‘Thor’ got his name. Ultimately the word can be traced back to Proto-Indo-European, the word then being ‘tene’.