Crease up Definition 1 of 3Pro Tip 1/3Spontaneous Reaction Used when someone can't help but laugh, often unexpectedly or in an uncontrolled way. When he slipped on the banana, I just creased up! Pro Tip 2/3Intense Amusement Shows a higher level of amusement than just smiling or chuckling. The comedian had the whole crowd creasing up. Pro Tip 3/3Informal Context Mainly used in casual situations among friends or in lighthearted settings. At family gatherings, my uncle's stories make us all crease up.