In explication for North Americans-
Nappy, nappie, noppy- of liquors, foaming, having a 'top'. OED
Nappy- said of a cloth with a 'nap', a rough surface. OED
Nappy- a disobedient horse. Etc.

Nappy, colloquial for napkin. Napkin in this sense- a rectangular piece of cotton or absorbant material used as a baby's undergarment by folding, drawing up between the legs and fastening at the waist. 19th c. OED
Diaper in this sense- A baby's napkin or 'clout'. OED
1596, Shakespeare Taming of the Shrew- "Let one attend him with a silver bason, ... Another have the ewer,... the third a diaper" OED