pig·gy·back 
(pĭgē-băk′)
(pĭgē-băk′)adv. & adj.
1. On the shoulders or back: ride piggyback; a piggyback ride.
2. By or relating to a method of transportation in which truck trailers are carried on trains, or cars on specially designed trucks.
3. In connection with something larger or more important: a tariff provision that came piggyback with the tax bill; a piggyback provision to a new piece of legislation.
n.
The act of transporting piggyback.
v. pig·gy·backed, pig·gy·back·ing, pig·gy·backs
v.tr.
To cause to be aligned with an issue, for example, that is larger or more important: "a $21.5-million federal grant to piggyback city and state subsidies" (New York).
v.intr.
To function as if carried on the back of another: "This reagent will piggyback onto an enzyme" (Seth Rolbein).
[Alteration of dialectal pig back, alteration of pickaback, pickback, pick pack : probably dialectal pick, to throw (variant of PITCH2) + BACK1 or PACK1.]