They made a calf in Horeb - Exodus 32:4. Probably in resemblance of the Egyptian god “Apis.” The image was made by Aaron out of materials furnished by the people, and at their request Exodus 32:1-3, so that it might be said to be the act of the people.
And worshipped the molten image - The word rendered “molten” is from a verb נסך nâsak - to pour, to pour out; hence, to cast, to found; and it means anything that is made by fusion or casting. This image was cast Exodus 32:4, and hence, this name is given to it.