When you are at work in your office looking up porn on the computer while stroking your cock under the desk.
You suddenly realise you are going to cum and don't want to cum over your hand or your trousers / shirt, so you quickly pull your handkerchief out of your jacket pocket or trouser pocket, and place it over your cock - like a tent. the handkerchief (hanky) will soak up your load, thus avoiding having embarrasing stained clothes.
"I was looking up fat cum bitches at work today, and had to cum a hanky - you wanna see it"?

"Eww no that's discusting"!
by Mr Hanky Chiefer July 5, 2010
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A Hanky or Handkerchief that is used frequently for masturbation.
damn Mandy is so fuckin'hot - I need to shoot a big load into my cum hanky
by hankie cuma March 17, 2008
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when a hanky is used as a turn on during masturbation / sex
or used as a target to ejaculate onto
Woman (unfolding her hanky onto mans stomach while masturbating him)
"Are you going to cum on my hanky baby?"
Man
"Yes..... Ahhh Ahhhhh Ahhhhhchooooo...." (man ejaculates all over womans hanky)

Or

Man "Dude did you just cum on my hanky?"
by My Hanky Cum April 27, 2008
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