More common in use might be “Shit-Kicker” which could refer to an oafish person, a farmer, a rube or to the boots.
That’s a new word to me, but I find that the OED knows it, and it appears to have originated independently in Australia and the US, with slightly different meanings:
1. Australian. A person who performs menial or unskilled jobs.
1950 ‘Thirty-Five’ Argot Sh-t kicker, a short-timer (often employed on sanitary work).
1953 S. J. Baker Austral. Speaks 172 Shitkicker, a ranker; ‘one of the boys’.
1970 B. Oakley Salute to Great McCarthy 168 Suburban shitkicker! Clerk!
1979 S. Weller Old Bastards I have Met 37 Syd was a mining engineer and Bill and Jack were two experienced miners and I was the shit-kicker.
2004 A. Nelson in S. Holdsworth & T. Caswell Protecting Future ix. 132 Realizing that I could get no interesting work without qualifications, I got sick of being a shit-kicker and went to university.
2. originally and chiefly U.S.
a. A tough or belligerent person; a ruffian; a troublemaker.
1954 B. Wolfe Late Risers 22 A shitkicker, you understand, is a gangly male who is all fists in the bunkhouse.
1967 L. Bruce Essent. Lenny 60 Showing his scar is beautiful. That’s just where he’s at. He’s a shitkicker.
1986 L. Server in N. Southern & J. A. Friedman Now dig This (2001) 8 He’s a shit-kicker..he’s a troublemaker.
1990 H. G. Bissinger Friday Night Lights vi. 126 A girl who everybody agreed was about the toughest shit-kicker in Odessa knocked another girl to the ground with a few punches.
2005 S. Evans Seaweed on Street (2008) iv. 67 I’ve seen these boys led astray, their heads turned by shiny motorcycles. Threw a leg over a hog and thought they were shitkickers and tough guys.
b. Chiefly depreciative. An unsophisticated person from a rural or remote area; a hick, a rube.
1961 J. Peacock Valhalla 121 I like Hank Williams… He was an American shitkicker.
1963 Amer. Speech 38 171 A few of the more colorful expressions appearing once are:..shit kicker, shit stomper, smash and swinger.
1969 L. Michaels Going Places 23 I was a city boy. No innocent shitkicker from Jersey.
1973 T. Crouse Boys on Bus i. ii. 40 Although he himself sometimes described his chain as a ‘bunch of shitkicker papers’, he was proud of his position as a national political writer and the dues he had paid to win it.
1975 W. Kennedy Legs 197 He saw the shitkicker’s cap, country costume, and he hated the man for wearing it.
1989 R. Banks Affliction xxii. 314 He could go to weddings or funerals..and not look like a hick, a woodchuck, a goddamned shitkicker from the hills of Cow Hampshire.
1990 S. Morgan Homeboy lii. 311 He tuned his radio to the shitkicker station..humming along to a chickenfried ‘White Christmas’.
2003 G. Burn North of Eng. Home Service (2004) iv. 114 And, hick that he was—carrot-cruncher, shit-kicker; he knew the words—Jackie had been taken in.
3. U.S. In plural. Large heavy boots or shoes, esp. with thick soles and reinforced toes; spec. cowboy boots.
1966 in Dict. Amer. Regional Eng. (2002) IV. 914/2 [Men’s low, rough work shoes] Shit-kickers.
1970 M. Thomas Total Beast 117 I remember King Kong had a convict on the ground one time, working his ass over with them sharp-pointed cowboy boots, them shitkickers, drawing back and kicking him all up in his ass.
1990 M. Brave Bird & R. Erdoes Lakota Woman (1991) viii. 117 Her cowboy boots were tipped with metal. She was showing them off: ‘These are my special shitkickers. I’m putting on war paint’.
2003 D. Gaines Misfit’s Manifesto iii. 57 Sandy-colored desert boot shit kickers, navy sport jacket, and white button-down shirt, Tommy was dressed.
So it looks likely that claykicker was invented for the script as a euphemism for, or in imitation of, the Australian sense of shitkicker. But that does seem an oddly downmarket insult to put into the mouth of 17th-century French prince.