Builder's itch is when someone (usually employed in a
engineering carrer, applies to people very active in creation communities yet not employed
in the field) has an unshakable "itch"
in the back of their mind compelling them to make or build something. Take it as a machinist building a "car" out of an old engine he had lying around and a children's peddal car
Tl:Dr - When a builder or creater has an excessive want to build/create something