<Verb> The act of blowing a joke or a rip, usually by sputtering out sentence fragments or by stuttering. This usually leads to laughter by one's friends and extreme embarrassment to the person who 'derailed.' The offender will stay very quiet for a long period of time following one of these outbursts.
"Man, you're so stupid when god handed out brains you thought he said trains...and you went...and got on...and went for a ride...wait....goddammit!!!"
"HAHAHAHAHAHAHAHAHA HOLY SHIT, you're retarded!! Nice derail, Jay!!"