The action of inserting a flacid or semi flacid penis into ones sexual partner using a thumb or finger in an attempt to engage in coitus. The action is usually performed after the consumption of a considerable amount of alcohol and is regarded as a token effort at sexual intercourse.
"I was absolutely twatted and knew that there was no way that I could possibly perform, despite the fact that she was absolutely stunning and horny as a goat. I knew that if we didn't have sex she would be upset but I was drunk as a sailor. Therefore the only possible course of action was to thumb in a softie.