From Wiktionary, the free dictionary

From side +‎ load, modelled on upload and download.

sideload (third-person singular simple present sideloads, present participle sideloading, simple past and past participle sideloaded)

1. (transitive, computing) 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.

   • 2010, Jim Cheshire, Using Nook:

     When you sideload content onto your nook, you'll find the items in My Documents.

2. (transitive, computing) To install an app on a mobile device by using unofficial channels, such as avoiding the use of an app store by obtaining it directly from the developer or downloading from a software repository.

   Synonym: jailbreak

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