"Piston slap" is when the skirt of the piston is dragging, or "slapping" on the side of the cylinder wall.
In an ideal situation the piston never touches the sides but is held away by the rings.
However, with age the clearance between the piston and wall opens up,
and causes the piston to wobble in the cylinder.
When the engine is cold it's worse.
Aluminum (the piston material) has a much higher expansion rate than iron,
and closes the gap when operating temperature is reached.
The newer engines have increased clearances to lower friction losses in order to boost gas mileage.
Seems like a lot of newer engines have this malady now-a-days.
WD-40 Reverend Grip-Shift
85 Bronco XLT, '69 351W, C6-Shift Kit, 9"-Detroit, Dana 44-True Trac, 2" Lift, 33X12.5s