"rod for measuring the depth of a liquid" (originally and especially the oil in a motor engine), 1927; see dip (v.) + stick (n.). For slang "penis" sense, see dip (n.2).
Entries linking to dipstick
dip v.
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."
stick n.
Old English sticca "rod, twig, peg; spoon," from Proto-Germanic *stikkon- "pierce, prick" (source also of Old Norse stik, Middle Dutch stecke, stec, Old High German stehho, German Stecken "stick, staff"), from PIE root *steig- "to stick; pointed" (see stick (v.)).
Meaning "staff used in a game" is from 1670s (originally billiards); meaning "manual gearshift lever" is attested by 1914. Alliterative connection of sticks and stones is recorded from mid-15c.; originally "every part of a building." Stick-bug is from 1870, American English; stick-figure is from 1949.
dip n.2
"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].