XXX

from WordNet (r) 3.0 (2006)

xxx
    adj 1: being ten more than twenty [syn: {thirty}, {30}, {xxx}]
    n 1: the cardinal number that is the product of ten and three
         [syn: {thirty}, {30}, {XXX}]
    2: (genetics) abnormal complement of three X chromosomes in a
       female

from The Collaborative International Dictionary of English v.0.48

xxx \xxx\, adj.
   The Roman number representing {thirty}; -- in the
   postposition, designating the thirtieth in a series.

   Syn: thirty, 30.
        [WordNet 1.5]

from The Collaborative International Dictionary of English v.0.48

xxx \xxx\, adj. [by reduplication of the "X" used in some
   movie-rating systems.]
   Very obscene or pornographic.

   Syn: X-rated, obscene, pornographic.
        [WordNet 1.5]

from Jargon File (4.4.4, 14 Aug 2003)

XXX
 /X.X.X/, n.

   A marker that attention is needed. Commonly used in program comments
   to indicate areas that are kluged up or need to be. Some hackers liken
   `XXX' to the notional heavy-porn movie rating. Compare {FIXME}.

from The Free On-line Dictionary of Computing (8 July 2008)

XXX

   /X-X-X/ A marker that attention is needed.  Commonly used in
   program comments to indicate areas that are {kluge}d or need
   to be.  Some hackers liken "XXX" to the notional heavy-porn
   movie rating.  Compare {FIXME}.

   [{Jargon File}]

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