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1.
adj. Bloated and obese. Derives from the state of a tick(parasitic insect) after it has gorged itself on a host mammal's blood. Not complimentary. I've heard it in the context of Australian slang, but I'm sure it's spoken all over the place.
"Christ! All that beer you've been guzzling latley has made you fat as a tick!"
by Lulu July 02, 2004
 
2.
Stuffed from eating after a big meal.
As in after Thanksgiving "I'm bout' fat as a tick"
by E-Rock December 07, 2004