first come first served

first come, first served

The first people present will be the first to receive something, often something that is available in limited quantities. The first hundred people will get a free T-shirt—it's first come, first served.
See also: first, serve
Farlex Dictionary of Idioms.

first come first served

COMMON You say first come first served to show that a group of people will be dealt with or given something in the order in which they arrive. There will be five buses, first come first served. Note: People often say that something will be done or given on a first come first served basis. Tickets will be allocated on a first come first served basis.
See also: come, first, serve
Collins COBUILD Idioms Dictionary, 3rd ed.

first come, first served

The prompt get first choice. This idea was stated by Chaucer (ca. 1386) in The Wife of Bath’s Tale, “Whoso first cometh to the mill, first grist,” and was cited as a proverb by Erasmus. An early reference with the exact modern wording dates from about 1545, in Henry Brinklow’s Complaynt of Roderick Mors. See also early bird catches the worm.
See also: first, serve
The Dictionary of Clichés by Christine Ammer
See also:
  • first come, first served
  • first in, best dressed
  • be first out of the box
  • have dibs on
  • have dibs on (something)
  • give (someone) the first crack at (something)
  • refusal
  • first refusal
  • have (the) first crack at (something)
  • get the first crack at (something)