Blow off is a British slang term usually used by children to mean "fart". This term was prevalent up until recently but its use seems to be declining and the bawdy American version which means to ignore, or snub, is taking over.
This is one of the reasons Americans are laughed at in Britain as it causes some humour in British households.
My older sister was on the phone to her boyfriend last night and I heard her blow off just as she was pretending to kiss him.
Hey these days you dont talk to nobody, you know what they call you down at Morgans Bar? huh? they call you Mr Blow off, yeah ! hey how about that.