“Maths” is the contracted form (https://www.imperial.ac.uk/brand-style-guide/writing/grammar...) of MATHematicS. It had been in use in English-speaking countries since more than a century ago. As a matter of fact, it was also used in the US during the 19th century. Yet, for some curious reason, the Americans had stopped using contractions from some time onwards. So, “maths” became “math” and “PhD” became “Ph.D.”.