After orgasm, if left in position, as the male organ softens, and the muscles of the vaginal walls haven't been stretched, in child birth, for example, at some point, the flacid penis will be ejected, not with an audible "snap" as the title would indicate, but with a reaction not found in post-parturition vaginas.
Early to mid 80s vernacular for an attractive young woman. Popular in Massachusetts and nearby regions of adjacent New England states. Often pronounced as "snappa".