Entry from OED Online

[< SMART a. + CASUAL a.] 

    A. adj. Designating or characteristic of (a style of) dress which is informal yet smart, esp. smart enough to conform to a particular dress code.
 
  1945 N.Y. Times 18 June 16/5 Slim, tall and athletic of build, she is the ‘shirtwaist and suit’ type. Perhaps that accounts in a measure for her understanding of what the smart casual look in dress requires. 1966 Times 16 Dec. 15/6 Smart casual wear..form [sic] the basis of a man's après ski wardrobe, but he should take more than a change of polo-necked sweaters. 1988 Courier-Mail (Brisbane) 27 Sept. 17/2 Jennifer is a 20-year-old Alternative, with short platinum hair jelled and sprayed into a cone, bright face, smart casual clothes and heavy worker's boots. 1996 World's Fair 11 Oct. 10/3 (advt.) Over 18s. Smart casual dress. Music to please all.
 

    B. n. This style of dress.
 
  1982 Financial Times (Nexis) 25 May III. 12 Resorts have a definition of: ‘smart-casual’ in the evening, that is, in shorts, jeans or T-shirts. 1995 S. NYE Best of ‘Men behaving Badly’ (2000) 4th Ser. Episode 3. 137/1, I thought we agreed on smart-casual for this evening. I think you're edging into casual-casual there. 2003 Church Times 14 Mar. 15/4 This..rather a trendy don, sporting what we might now call smart-casual.