A condition exhibited by a worn-out internal combustion engine when the piston rings no longer efficiently seal the combustion chamber, allowing compressed air and fuel to escape the cylinder and thus power is lost and emissions are increased. The engine also usually burns oil.
I was going to buy that 350 from Carl, but it had too much blow-by and needed to be rebuilt
1)Blow-by occurs when the explosion in your engine's combustion chamber causes fuel, air and moisture to be forced past the rings into the crankcase. Your engine's rings must maintain an excellent fit in order to contain the pressure.
2) Blow-by also occurs when somebody who REALLY needs to take a dump lets loose a fart so putrid and heinous, that it smells as if he just crapped his pants.