Side loading is the process of installing applications from unofficial sources, often bypassing standard app stores. This increases the risk of installing malicious software, such as a rootkit, which is a type of malware designed to provide persistent privileged access while hiding its presence.
Reference:
CompTIA Security+ SY0-701 Official Study Guide, Domain 2.1: "Side loading applications from unofficial sources can introduce malware, such as rootkits, to the system." Exam Objectives 2.1: "Compare and contrast different types of threats."