Oral sex performed upon a female. Usually used to refer to a recipient who has an enlarged clitoris. Shellatio (pronounced she-latio) is a variation of 'fellatio'. The act itself is more similar to stimulating a male than an average female, hence the similarity in the name.
"I thought it'd be great to go down on Betty, but when I finally saw her gigantic clit, I realized I'd better be ready for some shellatio!"