First in, best dressed.
I believe it originated from an old Latin proverb...
`prior tempore, prior jure.'
[First in time, first by right.]
[First come, first served.]
@`Webster's New Universal Unabridged Dictionary' Deluxe Second Edition. 1964 Simon & Schuster
There are many proverbial forms of the thought since it's Latin origin:
He who comes first, eats first.
- Eike von Repkow (fl. c. 1220)
Cited in `Bartlett's Familiar Quotations' 16th Edition
Whoso that first to mille comth, first grynt.
- Geoffrey Chaucer (c. 1342-1400)
@(c. 1390) `Wife of Bath's Prologue'
Cited in `The Concise Oxford Dictionary of Proverbs' 2nd Ed.
first come first serued.
- Henry Brinklow
@(1548) `Complaint of Roderick Mors'
Cited in `Brewer's Dictionary of Phrase & Fable' 1981
There are many ethnic versions of this proverb as well.
"First Come, best dressed" seems to have originated in Australia, maybe in the 19th century.