V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

UTSL
       Use The Source, Luke (telecommunication, Usenet, IRC)


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

UTSL
 //, n.

    [Unix] On-line acronym for ?Use the Source, Luke? (a pun on Obi-Wan
    Kenobi's ?Use the Force, Luke!? in Star Wars) ? analogous to RTFS (sense
    1), but more polite. This is a common way of suggesting that someone would
    be better off reading the source code that supports whatever feature is
    causing confusion, rather than making yet another futile pass through the
    manuals, or broadcasting questions on Usenet that haven't attracted wizard
    s to answer them.

    Once upon a time in elder days, everyone running Unix had source. After
    1978, AT&T's policy tightened up, so this objurgation was in theory
    appropriately directed only at associates of some outfit with a Unix source
    license. In practice, bootlegs of Unix source code (made precisely for
    reference purposes) were so ubiquitous that one could utter it at almost
    anyone on the network without concern.

    Nowadays, free Unix clones have become widely enough distributed that
    anyone can read source legally. The most widely distributed is certainly
    Linux, with variants of the NET/2 and 4.4BSD distributions running second.
    Cheap commercial Unixes with source such as BSD/OS are accelerating this
    trend.


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

Use the Source Luke
UTSL

    (UTSL) (A pun on Obi-Wan Kenobi's "Use
   the Force, Luke!" in "Star Wars") A more polite version of
   RTFS.  This is a common way of suggesting that someone would
   be better off reading the source code that supports whatever
   feature is causing confusion, rather than making yet another
   futile pass through the manuals, or broadcasting questions on
   Usenet that haven't attracted wizards to answer them.

   Once upon a time in Elder Days, everyone running Unix had
   source.  After 1978, AT&T's policy tightened up, so this
   objurgation was in theory appropriately directed only at
   associates of some outfit with a Unix source licence.  In
   practice, bootlegs of Unix source code (made precisely for
   reference purposes) were so ubiquitous that one could utter it
   at almost anyone on the network without concern.

   Nowadays, free Unix clones are becoming common enough that
   almost anyone can read source legally.  The most widely
   distributed is probably Linux.  FreeBSD, NetBSD,
   386BSD, jolix also have their followers.  Cheap commercial
   Unix implementations with source such as BSD/OS from BSDI
   are accelerating this trend.

   (1996-01-02)