What is the lay term for "otitis externa"?

Outer ear infection. I would call it an outer ear infection or an ear canal infection. Also sometimes referred to as swimmers ear.
Swimmer's ear. "itis" means inflammation. "oto" or something like that means ear. So otitis means inflammation of ear. Externa just refers to outer part of ear (canal & visible portion) which is inflammed. Otitis media is the more common middle ear infection (other side of drum) in kids. Otitis externa is more common in swimmers (surfers, too) who can't/don't dry their ears completely.