Definition of 'Dirty Float'
A system of floating exchange rates in which the government or the country's central bank occasionally intervenes to change the direction of the value of the country's currency. In most instances, the intervention aspect of a dirty float system is meant to act as a buffer against an external economic shock before its effects become truly disruptive to the domestic economy.
Also known as a "managed float".
Investopedia explains 'Dirty Float'
For example, country X may find that some hedge fund is speculating that its currency will depreciate substantially, thus the hedge fund is starting to short massive amounts of country X's currency. Because country X uses a dirty float system, the government decides to take swift action and buy back a large amount of its currency in order to limit the amount of devaluation caused by the hedge fund.
A dirty float system isn't considered to be a true floating exchange rate because, theoretically, true floating rate systems don't allow for intervention.