Wikipedia gives a literal description for Neckbeard:
Neckbeard (a.k.a. Neard) — similar to the Chinstrap, but with the chin and jawline shaven, leaving hair to grow only on the neck. While never as popular as other beard styles, a few noted historical figures have worn this type of beard, such as Nero, Horace Greeley, William Empson, Moses Mendelssohn and Richard Wagner.
but I've seen it used in non-literal contexts. For example, in the documentation for a programming related tool, it talks about Neckbeard configuration:
Skip this section unless you must know what every line in your shell profile is doing.
Onelook.com only gave a link to Wikipedia's literal description, and to Wordnik, which only listed examples of its use rather than defining the word. What does the term mean when not used literally?