Synthetic speech can be used to express uncertainty in dialogue systems by means of hesitation. If a phrase like “Next to the green tree” is uttered in a hesitant way, that is, containing lengthening, silences, and fillers, the listener can infer that the speaker is not certain about the concepts referred to. However, we do not know anything about the referential domain of the uncertainty; if only a particular word in this sentence would be uttered hesitantly, e.g. “the greee:n tree”, the listener could infer that the uncertainty refers to the color in the statement, but not to the object. In this study, we show that the domain of the uncertainty is controllable. We conducted an experiment in which color words in sentences like “search for the green tree” were lengthened in two different positions: word onsets or final consonants, and participants were asked to rate the uncertainty regarding color and object. The results show that initial lengthening is predominantly associated with uncertainty about the word itself, whereas final lengthening is primarily associated with the following object. These findings enable dialogue system developers to finely control the attitudinal display of uncertainty, adding nuances beyond the lexical content to message delivery.