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]

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]

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

The Jargon File (version 4.4.7, 29 Dec 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.


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

XXX

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

   [Jargon File]