Sound it out. "Erm." Have you ever made a noise like that, especially when you don't really know what to say and you're trying to find the words to fit?

Err...ummmm... (That's how you have probably heard it. "Erm" is the more 'compact' way to typing it out, just like "lol" is the more 'compact' way to signify laughing in a post.) ...yeah, that dress doesn't make you look fat at all!