When you're stuck introducing someone who isn't your boyfriend/girlfriend, but they're definitely not just your friend either. Contraction of the phrase "Umm" and "Uh"/"Umm... Uh..."
"Why Jeffrey it's so good to see you again! And who is this lovely young thing you brought with you?"
"S/he's my, umm... uh... My umm, uh... S/he's my umma."