thanks for the meal (slang)
A jocular, slangy twist on いただきます, used playfully before eating. The マンモス part is a humorous, meaningless addition.
See also: いただきます
Saying 'Itadaki mammoth!', we all started eating with a laugh.
A humorous alteration of いただきます, with マンモス (mammoth) added for playful effect. The exact origin is uncertain, but it likely arose as a casual, jocular expression among young people.