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1.
When hooking up with a really drunk girl and she passes out before you cum, turn her over and blow your load all over her ass crack. When it drys, it will seal her butt cheeks together and she will have to pry them open the next day - hence, sealing the envelope.
Man this girl was so drunk last night, I just had to seal her envelope.
by Andrew Schwartz October 01, 2003
 
2.
To lick a Vagina.
"I'll Seal the Envelope all Night"
by Envelope Sealer March 27, 2010