After the timeskip, Kid’s left arm appears to be entirely mechanical indicating that he lost his arm during the two year timeskip, as well as somehow connected to his nervous system, shown by the presence of several metallic wires embedded directly into his skin.
How he lost his arm is still a mystery but I think I have a theory on how that could have happened.
Sometime during the two-year timeskip, Kid attempted to invade Totto Land but was defeated and forcibly “escorted” from the territory.
At this point, Kid replaced his left arm with a large mechanical one.
Probably Kid was forced to spin Big Mom’s Roulette. The Roulette is a device utilized by Charlotte Linlin to decide judgement on those who challenge or desert her.
The vertically placed Roulette is divided into eight evenly sections, each of which are further divided by five concentric sections; each section alternate in black-and-white. The first inner concentric section has two of each symbol: heads, arms, legs, and a skull donning a crown. The second and fourth concentric sections are thin ones: they each have the number “10” on them. The third concentric section each have the number “100”. The outer sections each have the number “1000”.
Eustass Kid was offered to spin the Roulette but he resulted in the “arm” option. Probably a lucky roll when considering the alternatives.
*Theory by themitchellbrothers