"stupid person, eccentric person," 1920s slang, perhaps a back-formation from dippy. "Dipshit is an emphatic form of dip (2); dipstick may be a euphemism or may reflect putative dipstick 'penis' " [DAS].
1590s, "act of dipping," from dip (v.). Sense of "a downward slope" is by 1708. Meaning "liquid into which something is to be dipped" is attested by 1825, in 19c. especially "sweet sauce for pudding, etc.," also "juices and fat left after cooking meat." The sense "thick, savory sauce for dunking pieces of raw vegetables" (by 1962) is probably a modern re-coinage.
Old English dyppan "to plunge or immerse temporarily in water, to baptize by immersion," from Proto-Germanic *daupejanan (source also of Old Norse deypa "to dip," Danish døbe "to baptize," Old Frisian depa, Dutch dopen, German taufen, Gothic daupjan "to baptize"), related to Old English diepan "immerse, dip," and probably a causative of Proto-Germanic *deup- "deep" (see deep (adj.)).
Intransitive sense of "plunge into water or other liquid" and transferred sense "to sink or drop down a short way" are from late 14c. From c. 1600 as "to raise or take up by a dipping action;" from 1660s as "to incline downward;" from 1776 as "to lower and raise (a flag, etc.) as if by immersing."
late 14c., as a type of diving bird, agent noun from dip (v.). As "a ladle or long-handled utensil for drawing liquid," from 1783, chiefly American English. Also "a Dunker, an Anabaptist" (1610s); "one who dips snuff" (1870). As the popular U.S. name for the asterism known in Britain as The Plough or Charles's Wain, attested by 1833, so called for the arrangement of the stars. Compare Big Dipper.
1807, American English, "sauce, gravy; any thick liquid," from Dutch doop "thick dipping sauce," from doopen "to dip" (see dip (v.)). Used generally by late 19c. for any mixture or preparation of unknown ingredients.
Extension to "narcotic drug" is by 1889, from practice of smoking semi-liquid opium preparation. Meaning "foolish, stupid person" is older than this (1851) and may be from the notion of "thick-headed," later associated with the idea of "stupefied by narcotics."
Sense of "inside information" (1901) may come from knowing before the race which horse had been drugged to influence performance (to dope (v.) in this sense is attested by 1900). Dope-fiend is attested from 1896, "a victim of the opium habit."