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1.
The Sunday following a heavy drinking session on a Saturday night, usually including a late night club or other strenuous activity. The stun effect is usually caused by the consumption of large amounts of vodka and energy drink resulting in a fresh but disorientating feeling, like a hangover but altered due to the effect of excess stimulants still making their way out of your system on the Sunday.

Stunday could be worse if drinks include Absynth, overproof Rum or liquor with a high alcohol content.
Richard: “Are you coming out to walk the dogs in the park?”

Tom: “ahh man I don’t think I can. It took me 30 minutes to make a cup of coffee this morning, I’m having quite a stunday today I’m afraid. Ask me again in a couple of hours”
by allans1 December 20, 2012