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1.
It means that a person agrees with someone or something, but is too fucking lazy to say "Yes" or "I agree."
Person 1: "Say, those strawberries are scrumptious!"
Person 2: "Mmhm."
by Jason Cooper June 25, 2006
 
2.
Stand for Minger minger heavy minger.

Describes someone to be extremely ugly.
Pronounced "Mmhm" in conversation.
" WOW she was ugly "
"M.m.h.m"
by Dan Wiggarmen January 02, 2010