“Piggyback” has been around since at least the 16th century. The earliest forms of the term include “pick pack,” “pick back” and “pick-a-pack,” and make no reference of pigs. The common element in these early forms, “pick,” is an old English dialect word related to “pitch” meaning “to throw or place” (as we “pitch a tent” today). The “pack” was most likely the load carried, whether inert or human, so “pick-a-pack,” for instance, might mean to “pick (put) the load on the bearer’s back.” The use of “back” in some early forms reinforces this interpretation.
Around the 18th century, “pickaback” became the dominant form, but there was problematic. The “back” part was clear, but no one at that point understood where the “picka” came from. So through a process fairly common in language known as “folk etymology,” people replaced the part of the word that made no sense (“picka”) with one that sorta, maybe, kinda did (“piggy”). Alas… “piggyback.”
Figurative uses of “piggyback” are fairly recent, dating back just to the 20th century, and most of those have involved carrying one thing on another (e.g., a child on the back of an adult, trucks on flatbed railway cars and even space travel).
Today, piggyback refers to using someone else’s technology. Specifically, it means to hop on board someone else’s unsecured wireless network in order to access the Internet with your laptop or mobile device. For example, “let me stop in that hotel lobby, I may be able to piggyback onto their network without having to pay for the Starbucks hotspot.” Another example, “At the moment, Google and Yahoo are not charging consumers to piggyback onto their mapping engines.”