A strikebreaker. Someone who goes to work during a strike, possibly having to cross a picket line. May or may not be a union member. See also scab.
X is a blackleg. I saw him sneaking into the factory yesterday.
A very large penis, may be dark in color, but not necessarily. This word comes from a mix of "third leg" and the legend that people with dark skin have bigger penises then whiter people.
My blackleg was floppin' all over the place and hittin' my knees.