Used originally for people using the website goeyeball.com to price monitor items and save money during the recession of 2009. A more broad definition as someone who knows how to save and keeps an eye on sales.
banking term; used to describe a situation where you make a guestimate by looking at it (eg. a stack of change or wad of bills) and being precise about the amount.
Dude! That's $536.78.. whoa, I'm right! I totally eyeballed it!!!