A compound noun is a noun consisting of two or more words and that can be either written as one word like breakfast and headache, as two or more words like eye shadow or with a hyphen like tee-shirt and dry-cleaning.
The word "cupful," then, is a compound noun consisting of the noun "cup" which refers to small bowl-shaped container, and "ful" ("full") that means something containing as much or as many as is possible or normal, and it refers to "the amount held by a cup."
