1640s, "having no indication of time," from date (n.1) + -less. From 1923 as "free from engagements or appointments," from date (n.3).
Entries linking to dateless
date n.1
early 14c., "a period or stretch of time, a season, an age;" mid-14c., "time when something happened or will happen," from Old French date (13c.) "date, day; time," from Medieval Latin data, noun use of fem. singular of Latin datus "given," past participle of dare "to give, grant, offer" (from PIE root *do- "to give").
From late 14c. as "the part of a writing or inscription which specifies when it was done." The sense transfer from "given" to "time" is via the Roman convention of closing every article of correspondence by writing "given" and the day and month — meaning perhaps "given to messenger" — which led to data becoming a term for "the time (and place) stated." A Roman letter would include something along the lines of datum Romae pridie Kalendas Maias — "given at Rome on the last day of April."
Out of date "no longer in vogue" is attested from c. 1600.
-less
word-forming element meaning "lacking, cannot be, does not," from Old English -leas, from leas "free (from), devoid (of), false, feigned," from Proto-Germanic *lausaz (cognates: Dutch -loos, German -los "-less," Old Norse lauss "loose, free, vacant, dissolute," Middle Dutch los, German los "loose, free," Gothic laus "empty, vain"), from PIE root *leu- "to loosen, divide, cut apart." Related to loose and lease.
date n.3
"liaison at a particular time, by prearrangement," 1885, gradually evolving from date (n.1) in its general sense of "appointment." The romantic sense is by 1890s. Meaning "person one has a date with" is by 1900. Date-rape is attested by 1973.