A Moistie can be compared to that of the similar definition Stiffy, but in relation to a woman. generally the fluid is in a similar volume to Niagra Falls.
1: A cultured, sophisticated, knowledgeable, discriminating enthusiast who posts fair and balanced, unbiased, insightful reviews and constructive criticisms of various science fiction works on the Internet’s Moist Board.
A moistie is a rediculously HOT young girl (preferably between the ages of 16 and 16 and 1/2) who, while illegal to sleep with, can clean your house, water your lawn, make you dinner, etc. Moisties like to be around males 3-5 years older than they are, and often get 'played'. Every hot girl was once a moistie (See Moistie Alumni) or will be a moistie (See Moistie Cadet).
Hailing from a cool way of saying "wet behind the ears", some say the origin of 'moistie' traces back to 2000 in Los Alamos, NM.