I do not claim to know the origin of this particular term []{} but I have reasons to doubt the male anatomy hypothesis.
In the most vulgar kind of Japanese, []{} is sometimes added to a noun like a prefix to express the speaker's hatred or strong disrespect of the object. The nuance it carries is much worse than "good for nothing". For this purpose, is pronounced **** rather than the dictionary reading **** .
Examples: []{}(Diet member)[]{}(actor)[]{}(nation) []{}(bar), etc.
Thus, I would tend to assume that the term came by this route.