Green’s Dictionary of Slang

one who is suave or stylish in conduct or appearance, usu. a man; often with unfavourable sense: a slick but shallow or insinuating person.

1929		Phila. Inquirer 16 June n.p.: The ‘smoothie,’ a college man who has something which is difficult to describe. Whatever it is, he has it; and, the article goes on to say, ‘when one is entertaining a young lady it is always well to avoid a “smoothie,” lest he try to “smooth” ’em up’.
1932		J.L. Kuethe ‘Johns Hopkins Jargon’ in AS VII:5 336: smoothie—a very attractive girl; a dude. (Sometimes ‘smoother’ is used as the masculine form.).
1935		F. Wead Ceiling Zero Act II: You ought to have more sense than to fall for a smoothie like him.
1940		New Republic 17 June 819: The American people are tired of the smoothy in politics.
1949–51		in M. Daly Profile of Youth 120: In some schools to be rich is to be ‘a smoothie’.
1959		J. Thompson Getaway in Four Novels (1983) 23: Would a smoothie like Doc go out on a limb unless he saw a better one to grab.
1967		J. Hibberd White with Wire Wheels (1973) 199: Don’t tell me a smoothie like you is frightened!
1975		‘P.B. Yuill’ Hazell and the Three-card Trick (1977) 100: Twenty years it’s taken me to build this outfit, you think I’m giving away bits of it to young smoothies?
1982		N. Keesing Lily on the Dustbin 97: Dad says ‘You can’t tell a book by its cover’, and he’d prefer sister’s bete noir to a ‘smoothie’ any day.
1988		McGill Dict. of Kiwi Sl. 105/1: smoothiepuss a pretty woman. (Also ‘smoothy’.).
1993		Beatrice Dly Sun (NE) 21 Sept. 4/3: David Geergan, an old smoothie with years of press-stroking experience.
1997–2000		College Sl. Research Project (Cal. State Poly. Uni., Pomona) 🌐 Smoother (noun) Someone who is smooth; one who can persuade.
2004		(con. 1970s) N. ‘Razor’ Smith A Few Kind Words and a Loaded Gun 200: There were also punk-rockers, neo-mods, soul-boys, smoothies and teddy-boys.