Elastic is the rubber-based material that resumes its original shape after stretching or compression, and anything described as elastic is flexible.
In 1923 American inventor William H Spencer took elastic one step further, creating the elastic band — the Internet of its day for bored office workers seeking amusement. The properties of elastic have long lent themselves to metaphorical use, so that anything or anyone that's capable of adaptation or is flexible in its nature can be termed elastic.