好奇上网查了一下tramp gramp，下面引Urban Dictionary中的定义和解释，有点儿意思，好像也部分解答了你提的问题
“Tramp Stamp” is a derogatory term referring to a tattoo which a women places on her lower back. It is especially popular among women born in the late 70’s, 80‘s, and even early 90’s. Fair or unfair, these tattoos have a socially constructed connotation associated with them. These women are labeled as tramps, whores, or other derogatory sexually promiscuous terms. Although these are often bias generalized claims, there have been sociological studies done by the American Psychological Association, Federal Bureau of Prisons, and other demographic researchers showing strong correlative evidence associating tattoos with high risk behavior, illegal substance abuse and sexual promiscuity. These risk factors are greatest in the age range which these types of tattoos are gaining main stream popularity. Some have also jokingly stated that by 2050, the “tramp stamp” will be renamed the “gramp stamp”.