This paper provides a rationale for ambiguous language, understood as language generated according to an incompletely known communication rule. We consider a cheap talk game in which a (possibly ambiguity averse) sender (S) able to randomize according to unknown probabilities faces an ambiguity averse receiver (R). We show that under fairly general circumstances, there exist equilibria featuring ambiguous (i.e. Ellsbergian) communication strategies that allow both S and R to obtain a higher ex ante payoff than any non-Ellsbergian equilibrium. Ambiguity, by triggering worse-case decision-making, allows to shift R’s response to information towards S’s favorite action. R also benefits because equilibria featuring ambiguous communication strategies involve a larger amount of information transmission.