It comes from the Latin 'theodiscus', which comes from the germanic word 'theod', which means 'the people', 'das Volk'. So in the end 'tedesco' is a germanic word just like 'deutsch' and it literally means 'the language of the people'.

Why is Germany called Allemagne in French?

These people were called the Alemanni. So Allemagne is something like 'land of the Alemanni' just like France gets its name from a Germanic tribe, the Franks. English took Germany from the Latin word for the region, Germania.

Why is Germany called tyskland?

Tyskland (Scandinavian languages like Danish and Swedish), Duitsland (Dutch) and, of course, Deutschland (German) were terms that likely developed from the Proto-Germanic þeudō or Þeudiskaz, which respectively mean 'nation' and 'of the people'.