Android malware writers often utilize online malware scanners to check how well their malware can evade detection, and indeed we can find malware scan reports that were generated before the major outbreaks of such malware. If we could identify in-development malware before malware deployment, we would have developed effective defense mechanisms to prevent malware from causing devastating consequences. To this end, we propose Lshand to discover undiscovered malware before day zero, which we refer to as negative-day malware. The challenge includes scalability and the fact that malware writers would apply detection evasion techniques and submission anonymization techniques. Our approach is based on the observation that malware development is a continuous process and thus malware variants inevitably will share certain characteristics throughout its development process. Accordingly, Lshand clusters scan reports based on selective features and then performs further analysis on those seemingly benign apps that share similarity with malware variants. We implemented and evaluated Lshand with submissions to VirusTotal. Our results show that Lshand is capable of hunting down undiscovered malware in a large scale, and our manual analysis and a third-party scanner have confirmed our negative-day malware findings to be malware or grayware.