Etymology
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intrepid (adj.)
"unmoved by danger, undaunted," 1690s, from French intrépide (16c.) and directly from Latin intrepidus "unshaken, undaunted, not alarmed," from in- "not" (see in- (1)) + trepidus "alarmed," from PIE *trep-(1) "to tremble" (see trepidation). Related: Intrepidly; intrepidness (1620s).
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undaunted (adj.)
mid-15c., with reference to horses, "untamed, not broken in," from un- (1) "not" + past participle of daunt (v.). In reference to persons, "intrepid," it is recorded from 1580s.
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audacious (adj.)

1540s, "confident, intrepid, daring," from French audacieux, from audace "boldness," from Latin audacia "daring, boldness, courage," from audax "brave, bold, daring," but more often "bold" in a bad sense, "rash, foolhardy," from audere "to dare, be bold." In English, the bad sense of "shameless, unrestrained by propriety" is attested from 1590s. Related: Audaciously; audaciousness.

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valiant (adj.)
early 14c. (late 12c. in surnames), "brave, courageous, intrepid in danger," from Anglo-French vaylant, and Old French vaillant "stalwart, brave," present-participle adjective from valoir "be worthy," originally "be strong," from Latin valere "be strong, be well, be worth, have power, be able, be in health," from PIE root *wal- "to be strong." As a noun, "valiant person," from c. 1600. Related: Valiantly.
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clue (n.)

"anything that guides or directs in an intricate case," 1590s, a special use of a revised spelling of clew "a ball of thread or yarn" (q.v.). The word, which is native Germanic, in Middle English was clewe, also cleue; some words borrowed from Old French in -ue, -eu also were spelled -ew in Middle English, such as blew, imbew, but these later were reformed to -ue, and this process was extended to native words (hue, true, clue) which had ended in a vowel and -w. The spelling clue is first attested mid-15c.

The sense shift is originally in reference to the clew of thread given by Ariadne to Theseus to use as a guide out of the Labyrinth in Greek mythology. The purely figurative sense of "that which points the way," without regard to labyrinths, is from 1620s. As something which a bewildered person does not have, by 1948.

Thus hardy Theseus, with intrepid Feet,
Travers'd the dang'rous Labyrinth of Crete;
But still the wandring Passes forc'd his Stay,
Till Ariadne's Clue unwinds the Way
[Gay, "Trivia"]

The board game (originally Cluedo) was launched in 1949 in Britain.

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