he encontrado una foto, aunque la verdad, por el aspecto a mí no me ayuda demasiado.... en el diccionario le veo dos posibles sentidos:

Noun 1. flap - any broad thin and limber covering attached at one edge; hangs loose or projects freely; "he wrote on the flap of the envelope"
barndoor - an opaque adjustable flap on a lamp fixture; used in photography to cut off light from particular areas
coattail - the loose back flap of a coat that hangs below the waist
codpiece - (15th-16th century) a flap for the crotch of men's tight-fitting breeches
covering - an artifact that covers something else (usually to protect or shelter or conceal it)
earflap, earlap - one of two flaps attached to a cap to keep the ears warm
dag, jag - a flap along the edge of a garment; used in medieval clothing
overlap, lap - a flap that lies over another part; "the lap of the shingles should be at least ten inches"
pocket flap - a flap that covers the access to a pocket
fly sheet, rainfly, tent flap, tent-fly, fly - flap consisting of a piece of canvas that can be drawn back to provide entrance to a tent
tongue - the flap of material under the laces of a shoe or boot
2. flap - an excited state of agitation; "he was in a dither"; "there was a terrible flap about the theft"
dither, fuss, pother, tizzy
agitation - a mental state of extreme emotional disturbance
3. flap - the motion made by flapping up and down
flapping, fluttering, flutter
undulation, wave - (physics) a movement up and down or back and forth
4. flap - a movable piece of tissue partly connected to the body
animal tissue - the tissue in the bodies of animals

estoy haciendo una suposición, a ver si alguien te confirma o desmiente!

The only reference in English that I have found on Google to this is a recipe that requires either pork belly or pork flap. Secreto de cerdo is a comparable cut of meat - I think it is taken from the belly and it does look like a flap of meat - hope this helps

xx