expression
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
「いただきマンモス!」と言って、みんなで笑いながら食べ始めた。
Saying 'Itadaki mammoth!', we all started eating with a laugh.