Contraction of "fuck" and nah (being synonymous with "no") to form a emphasized statement of disagreement or answer to a query. Similar to saying "fuck no" but phonetically abbreviated.
"The internet sucks"
"fnah. the internet is the most baller shit ever."
Alternative (and slightly more pretentious) spelling of fnarr.
"Most ways of indirectly opening the PHP script would allow you to view it's contents. Maybe bofing the server would make it stop parsing, but don't take my word for it."
(See bof, boff.)