Being a Bella is to be clumsy and/or use an unnecessary amount of 'umm's and 'ahhs'. Originates from the books/movies The Twilight Saga by Stephanie Meyer and refers to Bella Swan.
Person 1: "Did you see Hannah earlier? She fell on the stairs."
Person 2: "Hannah's always falling over. She's such a Bella"
OR
Person 1: "I just- I- ummmmm- I need- ahhh- I just want to *gasp*"
Person 2: "Stop being a Bella and just spit it out!"
Being paticualry clumsy or saying 'ahh' and 'umm' more then is necessary. Originates from the clumsey stuttering mess that is Bella Swan in the Twilight Saga.
Person 1: "Did you see Serena today? She fall down the stairs!"
Person 2: "I know, she was being such a Bella"
Or
Person 1: "I just- ummm- I- *gasp*- I need- ahhhh"
Person 2: "Stop being a Bella and just spit it out"