I can't find much information about this expression. Obviously the meaning is clear (being lazy), but I was wondering about its origin. I just found some unconvincing sources on yahoo answers, but nothing more. Anyone knows something about it?
Here is a relevant, reliable source: http://www.accademiadellacrusca.it/it/lingua-italiana/consulenza-linguistica/domande-risposte/perch-fiacca-si-batte .
It's military jargon, like "battere la carica", "battere la ritirata", etc., and it originated in Piedmont at the end of the nineteenth century.
So, "battere la fiacca" means "being strenuously committed to … let laziness/idleness/indolence/tiredness prevail" (the contrary of a military attack).