mid-14c., "unintelligible talk, gibberish; chattering, jabbering," from Old French jargon "a chattering" (of birds), also "language, speech," especially "idle talk; thieves' Latin." Ultimately of echoic origin (cf. Latin garrire "to chatter," English gargle). Often applied to something the speaker does not understand, hence meaning "mode of speech full of unfamiliar terms" (1650s). Middle English also had it as a verb, jargounen "to chatter" (late 14c.), from French.
A special language belonging exclusively to a group, often a profession. Engineers, lawyers, doctors, tax analysts, and the like all use jargon to exchange complex information efficiently. Jargon is often unintelligible to those outside the group that uses it. For example, here is a passage from a computer manual with the jargon italicized: “The RZ887-x current loop interface allows the computer to use a centronics blocked duplex protocol.” (Seeslang.)