Smush ins are one of the coolest things to make. I remember when I was a kid, me and my sister would always try to defrost our hard ice cream in our dessert bowls into almost a thick milkshakey consistency before scoffing the lot. Then as we matured we realized that many flavors could be mushed in to improve the flavor of the rubbishy ice cream that our parents always used to give us.
So, from the word "mush" and the phrase "mushing it in" they became "smush ins." It was great going round the supermarkets as a kid secretly slipping possible smush ins into the trolley. Anything could be a contender—from maple syrup to bashed up chocolate bits, meringue, fruit—you name it, we would smush it! Winegums aren't so good though. But melted chocolate caramel bars are choooooooice.