Green’s Dictionary of Slang

(US) someone devoted to neatness and order.

1959		Seattle Times 16 Dec. n.p.: A neatnik is the opposite of a litterbug.
1960		N.Y. Times 3 Jan. 48: Seeing how you’re a Neatnik, you’ll be buying things like soap.
1968		‘Hy Lit’ Hy Lit’s Unbelievable Dict. of Hip Words 28: neatnick – Overly prim.
1977		C. McFadden Serial 86: Kate [...] confessed to being a neatnik.
2001		S. King Dreamcatcher 382: There was some sort of janitor inside his head, a neatnik.