When a group of people line up and an individual comes along and does something sexual or has sex with each one, one by one!

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A police technique in which a handful of people fitting a similar description are stood in a line together for a witness of a crime to see. The witness is then supposed to pick out the perp.

A queue of prostitutes, often of varying ethnicity and hair color, arranged linearly against a flat surface, normally a facade, for the purpose of offering patrons a very wide selection. The lineup is most commonly seen in the area that is common to various gentlemen's clubs, and the prostitutes are normally just getting off their gentlemen club shifts to make some extra money.

The biggest problem with the lineup is the patrons have to find somewhere nearby (outside) after making their selection. The lineup is most commonly found in cities like Atlanta, Miami, and Las Vegas.

the area beyond the breaking waves where surfers wait for a ride.

a) The performers that are going to be at a music festival.

b) When surfing, the waves break at a certain point. All the surfers are behind that point in a line, as to not get in the way when one is paddling for a wave that is about to break. That is the "lineup".

(Canadian, noun) Line or queue.