The English word “mattress” is borrowed into Setswana and nativised as “materase“. A bit of Setswana basic grammar first: Setswana nouns fall into various noun classes called “ditlhopha tsa maina“. Setswana grammar classifies singular and plural nouns into different noun classes. Nouns that begin with “ma-” are plural nouns falling into class 5. Amongst these are words like magapu, magala, matagwa, matlapa, etc. Their singular forms start with “le-“ and fall into class 4. They include legapu, legala, letagwa, letlapa, etc. Enough with boring grammar, now back to the mattress matter: when Setswana borrowed “mattress” from English and nativised it as “materase” the word was naturally slotted into the class of nouns that begin with ‘ma-‘, the plural noun class 5. The problem was that the English word “mattress” is not plural, it is singular, while materase because of its morphology was overnight turned into a plural noun in Setswana. So how did Batswana resolve this problem? They formed the singular form according to the rules of Setswana morphology. Leterase was formed! This in linguistics is what is known as backformation.