piggy back

[kujawski]

why is carrying someone on either your back or shoulders called a piggy back?

[arburgirl]

dunno, but I saw two cows "making love" in a field on the way to work this morning, I said to my partner," there's two cows over there having it off", and he said "bull" I said no it's true !

[kempie]

The OED has this explanation:

piggy-back, pick-a-back, advb. phr.

The primitive form was perhaps either a pick back or a pick pack; origin and form uncertain; the earliest examples have back, but the usual 17th century forms had pack, which still occurs in some dialects.

The evidence does not show whether the expression originally referred to a pack picked (pitched) on the back or shoulders, or to the back on which it is pitched; nor does it appear whether a pick answered to the French � pic �vertically, perpendicularly�, was due to reduplication as in tip-top, etc., or had some other source.