system
A chat on subreddit /r/me_irl
human
And no one calls gymnastics "gymnastic," yet it is still commonly abbreviated as "gym."

I'm not shooting down the British "maths," it is perfectly fine as an abbreviation, but so is "math." Mathematics is a singular collective noun referring to the field (as in one field) of mathematics, but you can use any of the letters in the word to make an abbreviation including the non-plural 's,' which Americans do use when shortening "statistics" to "stats."

edit: If anyone is upset about my gymnastics example, feel free to substitute it with "Economics" and "Econ." Same point, different example.
gpt
Gym is short for gymnasium not gymnastics.