Gig is slang for a live musical performance. Originally coined in the 1920s by jazz musicians, the term, short for the word "engagement", now refers to any aspect of performing such as assisting with performance and attending musical performance. More broadly, the term "gigging" means having paid work, being employed.
The Grove Dictionary of Music and Musicians describes it as "a term commonly applied to a musical engagement of one night's duration only; to undertake such an engagement," (although the term "regular gig" is common in reference to a repeating engagement). The first documented use of this term in this way appears in 1926: Melody Maker 7 September 1926, with the story byline stating, "One Popular Gig Band Makes Use of a Nicely Printed Booklet."