ha ha only serious [from SF fandom, orig. as mutation of HHOK, `Ha Ha
   Only Kidding'] A phrase (often seen abbreviated as HHOS) that aptly
   captures the flavor of much hacker discourse. Applied especially to
   parodies, absurdities, and ironic jokes that are both intended and
   perceived to contain a possibly disquieting amount of truth, or truths
   that are constructed on in-joke and self-parody. This lexicon contains
   many examples of ha-ha-only-serious in both form and content. Indeed,
   the entirety of hacker culture is often perceived as ha-ha-only-serious
   by hackers themselves; to take it either too lightly or too seriously
   marks a person as an outsider, a {wannabee}, or in {larval stage}. For
   further enlightenment on this subject, consult any Zen master. See also
   {{hacker humor}}, and {AI koans}.


Source: The Jargon File