n.: a title given to a humorous, intelligent, and interesting acquaintance with whom all matters can be discussed without fear of judgement and with certain encouragement, and who will also engage in sexual intercourse if prospects of getting anyone else are found to be too dim. Abbr.: S.M.
"Daniel and I agreed that if neither of us is married by age forty, he'll be my sex muffin."
"My name is Sophie Plesniak, S.M."