My guess (maths-s?). I suspect it is not a question of singular or plural, but rather they are shortened forms of "mathematics" which has an s at the end.
Indeed. The ending -ics is unusual, because despite the -S that generally identifies a plural noun, words ending in -ics are construed as singular.
Moreover, although words ending in -ics often take the definite article (the physics of black holes, the basics of string theory, the politics of jihad), they never take the indefinite article (no such phrase as "an aeronautics," "a mathematics," or "an acoustics").
The origin of the suffix and its various forms is murky, but it seems that "-ic" was common prior to the 1500s. At that point a renaissance of Greek scholarship brought attention to the Greek suffix -
ikos which was (blessedly) contracted to "-ics." This sheds some light on the phenomenon that older words like "arithmetic" have no -S, while neologisms like "semiotics" do.
Dictionary.com, which is an American website but nonetheless a very complete one, says only that we condensed "mathematics" into "math" by truncating the last three syllables, while the British decided to retain the -S, and it offers no explanation.
But it also notes that while "math" is treated as a singular noun in American English, on the other side of the Atlantic proper usage allows both singular and plural: "Math was difficult my wife" and "Maths were..." are both allowed.