It means I don't know the words to a song
It denotes a moment of contemplation and thoughtful consideration...
The longer the moment,..the more "mmmm"s.. (h--headspace...m-moment)
The same as humming under your breath but it seems not to be typed correctly as Humm might serve better as a short version of the word ****** .
I don't know but my refrigerator sings it all day long.
its the typed version of thinking...
It means your avatar is yummy.
I will have to ponder that one.
It can mean I was deep in a conversation with myself and you butted in.
I use it when I'm thinking about something.