A diesel dyke is a term used in some lesbian circles to refer to a lesbian who has a masculine or "butch" look. The term first appeared in Urban Dictionary in 2004, and is said to be derive from the notion that since diesel dykes tend to be bigger butch women, moving them would require a powerful diesel engine. This, of course, if pretty derogatory and fatphobic language. In the gay community, the term has taken on a more positive connotation and is more often assumed to be a lesbian who is into motorcycles, cars or mechanics (or just looks like they are!)
Another term for diesel dyke is bulldagger.