MT = "empty". Say it aloud. :) GG = slang word for horse; you won't hear it much nowadays, but maybe from old men at the horse races ("put a fiver or two on the GG's" = place some bets on the horses").

Evie and Peachey are right. But I think your daughter is a genius. She should send that suggestion to Cambridge and Oxford.

I heard this one before from a British co-worker. M.T. stands for 'empty'. In the United Kingdom, GG is a nickname for a horse probably because when you ride a horse, you say 'giddy up, giddy up'. So MTGG stands for empty horse.

"MT" is simply "empty" and GG is defined by Miriam Webster as "British, informal: a horse; used especially by children or when referring informally to horse racing. So, MTGG is a hungry horse.