SMUG is a 4 letter word that starts with S.

1. \Smug\, a. [Of. Scand. or Low German origin; cf. LG. smuck,
   G. schmuck, Dan. smuk, OSw. smuck, sm["o]ck, and E. smock,
   smuggle; cf. G. schmuck ornament. See {Smock}.]
   Studiously neat or nice, especially in dress; spruce;
   affectedly precise; smooth and prim.
   
         They be so smug and smooth.              --Robynson
                                                  (More's
                                                  Utopia).
   
         The smug and scanty draperies of his style. --De
                                                  Quincey.
   
         A young, smug, handsome holiness has no fellow. --Beau.
                                                  & Fl.
   
   

2. \Smug\, v. t. [imp. & p. p. {Smugged}; p. pr. & vb. n.
   {Smugging}.]
   To make smug, or spruce. [Obs.]
   
         Thus said, he smugged his beard, and stroked up fair.
                                                  --Dryton.