The Jargon File (version 4.4.7, 29 Dec 2003):

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 koan.


The Free On-line Dictionary of Computing (30 December 2018):

ha ha only serious
HHOS

   (SF fandom, originally as mutation of HHOK, "Ha Ha Only
   Kidding") A phrase (often seen abbreviated as HHOS) that aptly
   captures the flavour 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.

   The Jargon File 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 AI koan.

   [Jargon File]