It is in the second chapter that Rip Van Winkle awakens after having drank a flagon, a large bottle of liquor, with the English explorer Henry Hudson and his crew high up in the Catskill Mountains. After having drank the liquor, he fell asleep for 20 years. After he wakes up, he "rose to walk and found himself stiff in the joints, and wanting in his usual activity." Van Winkle's first thought is that having slept in the mountains did "not agree with [him]." The mountains are very damp and the ground is very hard. Hence, he blames his stiffness on his rheumatism.
Historically, the term rheumatism was used to refer to any stiffness in the joints or to any pain related to bones or bone connective tissue. However, today, we no longer use it as a medical term and instead speak of it in terms of rheumatoid arthritis, or simply arthritis. It's especially understood that changes in barometric pressure can cause pain; hence, any cold and wet weather, like Van Winkle experienced while asleep in the mountains, will cause joint pain.