"Get those street-Arabs out of my shop and into the flop-house where they belong!"
Angry shopkeep, c. 1890
Any house/apartment/frat house/trailer/etc. which is used for individuals to crash (sleep, chill, hang out, lurk, etc.) for a period of time. In order to "crash", one must not actually live there (e.g. have their name on the lease, own said flophouse, etc.). Flophouses are typically used by college students, drug addicts, transients, vagrants, or other unsavory characters.
A flophouse may also be the home owned by the parents of a college student, who after being away at college, wishes to live as if he/she is still at college (e.g. staying out all night, returning just to crash at mom and dad's house, then leaving again), rather than conforming to the pre-college rules.
Dude: (Talking to someone passed out face first on the floor of his college apartment) Hey man, who the hell are you? What do you think this is, a flophouse??
Bro: Sorry dude, I must have passed out for a minute.
Mom: Where have you BEEN for the last three days? You come home just to sleep?? What do you think this is, a flophouse??!!
Son: What's your problem? I'm an adult now!
A house in which one might flop.
Man 1:What the hell are you doing in my office!
Man 2:Uhhh, we rented this place out.
Man 1;I dont now about that, this aint no fucking flophouse you can just sleep in!
Another word for "whorehouse."
Sally works at the flophouse to make ends meet financially.