The expression first come, first served began life as a proverb having the same sense as the early bird catches the worm.
Both proverbs are admonitions against dawdling.
The proverb was adopted by shopkeepers to convey the idea that customers would be served in the order of their arrival. In case of limited quantities, latecomers would be out of luck. And if the local squire got there after the char lady, he’d have to wait his turn.
The expression has become so common in modern times that the abbreviation FCFS and even Fcfs is seen in advertising and on ticket-selling sites.
Because the expression originated before the 1900s when the idiom changed, modern speakers and writers have trouble with the usage.
A common error is to write the phrase as “first come, first serve.” The confusion arises from thinking that “come” is the same kind of verb form as “serve” and that they’re supposed to match.
One way to look at it is to think of first come, first served as an elliptical form of the first to come will be the first to be served.
Another is to recognize come as a past participle or adjectival verb form. Consider:
First seen, first treated.
First gone, first missed.
In first come, first served, “come” functions as an adjective. It’s not a common usage these days, but I actually found a contemporary example in a song written by Steeleye Span, a British electric folk band. It’s from their 2004 album They Called Her Babylon:
some said, “give him the beef, the beef,”
some said, “give him the bone.”
and some said, “give him nothing at all
but let the beggar roam.”
then up and spake the new-come lord,
a saucy word spoke he,
“pass round the cup, let my rival sup,
then send him on his way.”
Confusion about the expression involves punctuation as well as spelling.
Bookings must be made by midnight 22nd June, so hurry, spaces are limited and available on a first come first served basis.
Free flights from British Airways for small businesses looking to export. There are 4,000 up for grabs on a first come, first served basis.
Delta Airlines accepts pets on a first-come, first-serve basis.
NEWCASTLE United is set to be sold on a “first-come, first-served” basis.
I vote for the unhyphenated, unquoted comma version:
The new phones will be sold on a first come, first served basis.
Of course the problems of verb form and punctuation can be avoided entirely by going with FCFS.