"The car ain't running" is a term used to describe when somebody's joke is severely unfunny, to the point nobody laughs.
Whenever someone says an unfunny joke, you can say it to tell them that it was not funny.
*Someone tells an unfunny joke*
"Hey bruh, the car ain't running."
When everyone and their mother can tell you have feelings for someone and they clearly have them back but you're both too scared to admit it so you're stuck in that awkward "idk if we're talking" stage.
"Yeah, Ryan is definitely your boo who ain't your boo until he finally grows a pair to admit he likes you."
"Jake is your boo who ain't your boo cause you're too dang scared to tell him you like him."
A "modern day" variation on the classic "I'll be able to call David Rockefeller a tramp" saying, and used to humorously refer to your having made a modest profit and/or unexpectedly gained a few bucks extra on a difficultjob you recently completed.
Cool hunk #1, seeing his buddy --- who had jokingly told him earlier that morning that he was "wearing himself out getting rich" by driving here and there around town for a couple hours to give several people car-rides for a few bucks apiece --- feverishly lugging humongous bundles of bagged returnable-containers along a hotdusty highway: Gettin' too rich there, Dude?
Cool hunk #2: Yeah, fer sher, Pal --- I declare, I'm gettin' so gol-durned filthy-rich that I can hardly even carry all my vast wealth around! Ethan Couch ain't got nuthin' on me!