Internet borrowing, is in short, downloading something from the Internet that would otherwise cost money. Such as a book, or movie.

Usually this has the intent of deleting the content after being used/read/watched. This is usually used to avoid using the words download, pirate, or torrent, in online discussions to avoid bot based data mining techniques.

Alternate use is borrowing the use of someones Internet connection, usually without permission.

(noun) A successful type of theft which results in an acceptable, impressive and rewarding payday for the protagonist.