I remember somebody asking this question in comments. I could not locate the answer at that time in the books that I referred to at that time.
Today I came across the issue in Inorganic Chemistry by Dr.O.P.Tandon, 2007 edition, page 120.
Molecules haivng a multiple bond between atoms of dissimilar electronegativity act as Lewis acids. Typical examples of molecules belonging to this class of Lewis acids are CO2, SO2 and SO3. Under the influence of attacking Lewis base, one π-electron pair will be shifted towardsmore negative atom.
In page 128 question (viii) is more direct.
So# has acidic behaviour. Explain
Hint:Sulphur atom is attached with three very highly electronegative oxygen atoms. As a result, electrons are drawn towards oxygen atoms and S-atom remains wiht electron density and thus has the tendency to accept electrons. So it acts as a Lewis acid.