'3aja'it 3a Fassou7" is an expression which literally means "Fassou7 got crowded".  Fassou7 is an area in the Ashrafieh district of Beirut, previously (70's and 80's) known for its small little streets always jammed with cars and pedestrians.  The expression is used metaphorically to simply mean "It's getting crowded in here" whereever you are.  So you might be in the middle of shopping mall in Washington DC, you walk into a shop buzzing with people, and you would go "wow, 3ej'a 3a Fassou7".

Of course today, most roads in a metropolitan area such as Beirut are jammed.  But nevertheless, the expression stuck.