Definition of piggyback

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piggy bank >

pig·gy·back    (pg-bk) KEY 

ADVERB:
&
ADJECTIVE:

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.

NOUN:

The act of transporting piggyback.

VERB:
pig·gy·backed, pig·gy·back·ing, pig·gy·backs
VERB:
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).

VERB:
intr.

To function as if carried on the back of another: "This reagent will piggyback onto an enzyme" (Seth Rolbein).

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ETYMOLOGY:
Alteration of dialectal pig back, alteration of pickaback, pickback, pick pack : probably dialectal pick, to throw (variant of pitch²) + back¹ or pack¹