Undergraduates are often advised to expand their discipline-specific vocabulary in order to cope with the rigor of reading and writing academic texts. For the students studying in English-medium settings, it is in their interest to acquire vocabulary in their disciplines as this is vital to academic success. While students recognize the importance of discipline-specific vocabulary acquisition, they are typically unable to do so due to their heavy work on learning and assessment tasks. With the advent of mobile technology, one way to address this problem is to create vocabulary apps. To meet this need, this paper presents a gamified, discipline-specific vocabulary learning app "Books vs Brains@PolyU." It was developed to help busy undergraduates to build a repertoire of the vocabulary across seven disciplines. We explored students' perceptions and evaluation of this tool via interviews with sixteen student participants of the project. The results show that students found the tool useful and motivating, indicating the effectiveness of the app in helping busy undergraduates to build and expand their knowledge of discipline-specific vocabulary.