verb To transfer data between two local devices, rather than to or from a remote device.
I'm going to sideload my computer's address book onto my mobile phone.
verb To install an app on a mobile device by using any app store that doesn't come preinstalled like Google Play or Apple App Store. Or installing an app from an installation package. (.apk/.ipa)
Developer: I'm not paying 25$ to Google to put my free app on Google Play. User: I'm fine sideloading it from F-Droid. Thank you for the free app!