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1.
From Puerto Rican Spanish, it literally means "small horse for riding or driving" usually translated in English as “mare”, “nag” or “pony.” Etymologically, it is thought to come from the archaic English word to designate horses for riding: "hackney", also the name for the northeast area in 13th century London where horses formally grazed.
A “jaca” is not useful to conduct any work except for riding (in this sense, it is less useful than a mule). Therefore, it is often used as a derogatory term to refer to someone who is incapable of completing a simple task, even a menial one; someone who is useless, worthless, a moron, an idiot. Although not technically correct, it is also used as a synonym to "yegua" (mare). Another synonym, but whose usage is more limited to the Hispanic population of South Florida, is “valeria”, probably from someone’s “jaca” who was given the name “valeria.”
This woman is so annoyingly stupid, she is "una jaca", "una yegua", "una valeria".
by Cabro Pepe August 15, 2008
 
2.
From Puerto Rican Spanish, it literally means "small horse for riding or driving" usually translated in English as “mare”, “nag” or “pony.” Etymologically, it is though to come from the archaic English word to designate horses for riding: "hackney", also the name for the northeast area in 13th century London where horses formally grazed.
A “jaca” is not useful to conduct any work except for riding (in this sense, it is less useful than a mule). Therefore, it is often used as a derogatory term to refer to someone (especially a woman) who is incapable of completing a simple task, even a menial one; someone (usually a woman) who is useless, worthless, a moron, an idiot. Although not technically correct, it is also used as a synonym to "yegua" (mare). In South Florida, a “jaca” is also referred to as “valeria”, probably from someone’s “jaca” who was given the name “valeria.”
This woman is so annoyingly stupid, sh is "una jaca", "una yegua", "una valeria".
by Cabro Papito August 14, 2008
 
3.
n.
1. An acronym for the Jewish Agnostic Communist American Students organization, pronounced, simply, jack-ass.
2. Shaun.
Shaun is the only member of JACAS
by Shaun Rosten May 10, 2006