A shmaga is a s'more in which you replace the Hershey's chocolate with a Reese's Peanut Butter Cup
Dominic: Hey Lauren, do you want to go make some shmagas?!
Lauren: Do you have the Gram crackers? Marshmallows? Reese's peanut butter cups?
Dominic: Check, check and check!
Lauren: then what are we waiting for!
As versatile as the F-word, shmaga has a number of definitions, though it is mostly used as an adjective.
Adjective: He is such a shmaga.