1941 (typically 4 p.m. to midnight), from the notion of "facing both ways" between day and night shifts; see swing (v.) + shift (n.).
Entries linking to swing-shift
swing v.
Old English swingan "beat, strike; scourge, flog; to rush, fling oneself" (strong verb, past tense swang, past participle swungen), from Proto-Germanic *swengwanan (source also of Old Saxon, Old High German swingan, Old Frisian swinga, German schwingen "to swing, swingle, oscillate"), which is of uncertain origin and might be Germanic only.
The meaning "move freely back and forth" is first recorded 1540s. Transitive sense "cause to oscillate" is from 1550s. Sense of "bring about, make happen" is from 1934. Sense of "engage in promiscuous sex" is from 1964; earlier, more generally, "enjoy oneself unconventionally" (1957). Related: Swung; swinging. Swing-voter "independent who often determines the outcome of an election" is from 1966.
shift v.
Middle English shiften, from Old English sciftan, scyftan "arrange, place, put in order" (a sense now obsolete), also "divide, separate, partition; distribute, allot, share" (now obsolete or provincial), from Proto-Germanic *skiftan (source also of Old Norse skipta "to divide, change, separate," Old Frisian skifta "to decide, determine, test," Dutch schiften "to divide, turn," German schichten "to classify," Schicht "shift"). This is said to be related to the source of Old English sceadan "divide, separate" (see shed (v.)).
By c. 1200 as "to dispose; make ready; set in order, control," also intransitive, "take care of oneself." Thus "manage to succeed, make out a livelihood" (as in shift for oneself, 1510s; also compare makeshift).
The sense of "to alter, to change" appeared by mid-13c. (compare shiftless). Also from mid-13c. in the transitive sense of "remove and replace with another or others," originally especially of clothing, hence "put on and replace one's clothes" (c.1400).
From c. 1300 as "to go, move, depart; move (someone or something), transport" as from one place or position to another. The meaning "change the gear setting of an engine" is from 1910; to shift gears in the figurative sense is from 1961. Related: Shifted; shifting.