Don't blow it, man! You're so stupid, I know you're going to blow it!
A "blow it" is someone who constantly messes up any good situation.
They do this from greed, lack of ethics or just a plain lack of common sense.
Also, know as a dumb ass, bumbler, loser, fool, worthless fuck, etc.
"Geez, Reiner is one worthless blow it. He had a beautiful wife and child at home only to lose them from fooling around with a bunch of women."
1) to seriously fuck up a situation
2) to suck a given dick
3) to expel air out of the mouth toward an object
1) Liz really blew it when she started using her teeth while blowing it.
2) "Well, I'm on my knees. What should I do now?"
3) "I actually meant SUCK it, ya dumb bitch (SMACK!)"