The "flip side" was originally the term for the B side of a 45 rpm record. Forty or fifty years ago, "single" 45 rpm records would have the title track on Side A and a lesser known track on Side B. You had to flip from Side A to get to the second track, hence the "flipside" of the album.
As sung in "Off He Goes" from Pearl Jam's album "Yield".
"Said he'll see me on the flipside... of this trip he's taken for a ride, He's been takin'... too much on... off he goes with his perfectly un--kempt clothes."