A fat, or "thick" person, most often in refrence to potential sexual attraction. Generally the derogitory label attributed to a person, or type of person, towards which someone is typically aroused by. Thickins are usually, but not exclusively, female.
Alternately spelled "thickun," espc. south of the Mason-Dixon line.
Holy crap she's a thickin.
Hold on, are you talking about the girl that sits next to you in computer class? You think she's hot? Jon...please tell me you're not into the thickins.