The weakest currency in the world is the Venezuelan Bolívar. The value of the Venezuelan Bolívar is so weak that 50,000 units are worth less than one US dollar. This has consistently been the case, due to the hyperinflation in the South American country. The weakest currency of all times was the Zimbabwean dollar, for which 35 quadrillion units were equivalent to 1 US dollar. The Zimbabwean dollar was demonetized in 2015. 

35 quadrillion = 35,000,000,000,000,000