A cubit is a measurement based on the distance from one’s fingertips to the elbow... approximately 18”. Since this is somewhat variable, the ‘official cubit’ is somewhat open for interpretation and varied from culture to culture.
Yeah; I know what a cubit is. I was just adding on to Post #2 by dfwgator, which I assumed was part of a Bill Cosby skit back in the ‘60s about Noah’s Ark. Two lines were, “How long can you tread water?” and “What’s a cubit?”