Luffy’s rubber-like stretching powers are a result of Oda’s desire for the fighting style to be silly, so that no matter how tense a situation gets, the reader can relax without becoming stressed out.
The Devil Fruit Luffy gained his abilities from was originally called the Gomu no Mi (fruit of gum), but after Oda’s editor pointed out that a real Gomu no Mi existed, the name was changed to Gum-Gum Fruit (Gomu Gomu no Mi?, fruit of gum gum).