Having a quick sexual encounter, without foreplay.
Where one engages in casual sex, confined to a specific sexual act. Usually this involves a hand job, or other form of masturbation, where one party brings another to orgasm, quickly and without any other associated activity, such as kissing, hugging, or even preliminary acts, such as banal dialogue.
Usually a casual meeting of two people, where there is little discussion or dialogue, other than cursory discussion of what is being made available.
To solicit another, usually a stranger, for immediate sexual gratification, usually meaning a hand job, or a blow job. This is accomplished on the spot, or in adjacent locations such as in a public washroom stall, in a back alley or in a grove of trees, bushes.
It does not necessarily involve one being totally undressed either, and can be simply a person putting their hand down another's pants, and stroking the penis till the person ejaculates. After, the person simply moves on, not always requiring any other returned favors.
Practice (Associated Acts): Can also be found in soliciting spots, where one engages a male prostitute for masturbation or even anal intercourse. This is where a person is hired, to perform a specific sexual act, without any other preliminary discourse.
Anal intercourse is performed where the one simply lowers the pants, bends over, and the other penetrates them for a short period of time, usually till ejaculation.
Time involved is the over riding factor in a quickie.
Noteworthy: Not a common activity found among Lesbians, just generally among gay males.