Etymology
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five (adj., n.)

"1 more than four; the number which is one more than four; a symbol representing this number;" Old English fif "five," from Proto-Germanic *fimfe (source also of Old Frisian fif, Old Saxon fif, Dutch vijf, Old Norse fimm, Old High German funf, Gothic fimf), from PIE root *penkwe- "five." The lost *-m- is a regular development (compare tooth).

Five-and-ten (Cent Store) is from 1880, American English, with reference to prices of goods for sale. Five-star (adj.) is from 1913 of hotels, 1945 of generals. Slang five-finger discount "theft" is from 1966. The original five-year plan was 1928 in the U.S.S.R. Five o'clock shadow attested by 1937.

[under picture of a pretty girl] "If I were a man I'd pay attention to that phrase '5 O'Clock Shadow.' It's that messy beard growth which appears prematurely about 5 P.M." [advertisement for Gem razors and blades in Life magazine, May 9, 1938]
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high-five (n.)
1980, originally U.S. basketball slang, 1981 as a verb, though the greeting itself seems to be older (Dick Shawn in "The Producers," 1968). From high (adj.) + five, in reference to the five fingers of the hand.
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fiver (n.)
1843, "five-pound note," from five + -er.
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fivefold (adv.)
1570s, from earlier use as an adjective, from Old English fiffeald (adj.); see five + -fold.
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pentameter (adj.)

"consisting of five metrical feet," 1540s, from French pentametre, from Latin pentameter, from Greek pentametros (adj.) "having five measures," from pente "five" (see five) + metron "measure" (from PIE root *me- (2) "to measure"). As a noun from 1580s, "a verse line of five feet;" in ancient prosody "a dactylic dipenthemimeres or combination of two catalectic dactylic tripodies" [Century Dictionary].

The verses we have hitherto examined may be constructed at pleasure of any kind of metre—dactyl, troche, iamb, or anapest. But all at once, we now find this liberty of choice refused. We may write a pentametre verse in iambs only. A most notable phenomenon, significant of much more than I can at present understand,—how much less explain .... [Ruskin, "Elements of English Prosody, for use in St. George's Schools," 1880]
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tooth (n.)

Old English toð (plural teð), from Proto-Germanic *tanthu- (source also of Old Saxon, Danish, Swedish, Dutch tand, Old Norse tönn, Old Frisian toth, Old High German zand, German Zahn, Gothic tunþus), from PIE root *dent- "tooth." Plural teeth is an instance of i-mutation.

The loss of -n- before spirants is regular in Old English, Old Frisian, and Old Saxon: compare goose (n.), five, mouth (n.). Also thought, from stem of think; couth from the stem of can (v.1); us from *uns.

Application to tooth-like parts of other objects (saws, combs, etc.) first recorded 1520s. Tooth and nail as weapons is from 1530s. The tooth-fairy is attested from 1964.

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*penkwe- 

Proto-Indo-European root meaning "five."

It forms all or part of: cinquain; cinque; cinquecento; cinquefoil; fifteen; fifth; fifty; fin (n.) "five-dollar bill;" finger; fist; five; foist; keno; parcheesi; penta-; pentacle; pentad; Pentateuch; Pentecost; pentagon; pentagram; pentameter; pentathlon; Pentothal; Pompeii; Punjab; punch (n.2) "type of mixed drink;" quinary; quincunx; quinella; quinque-; quinquennial; quint; quintain; quintet; quintile; quintessence; quintillion; quintuple.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit panca, Greek pente, Latin quinque, Old Church Slavonic pęti, Lithuanian penki, Old Welsh pimp, Old English fif, Dutch vijf, Old High German funf.

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quinque- 

before vowels quinqu-, word-forming element from classical Latin meaning "five, consisting of or having five," from Latin quinque "five" (by assimilation from PIE root *penkwe- "five").

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quinquangular (adj.)

"having five angles," 1650s, from Late Latin quinquangulus "five-cornered," from quinque "five" (from PIE root *penkwe- "five") + angulus "angle" (see angle (n.)). Quinquangle (n.) "pentagon" is attested from 1660s.

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quinary (adj.)

"pertaining to the number five; divided in a set of five," c. 1600, from Latin quinarius "consisting of five, containing five," from quint "five each" (from PIE root *penkwe- "five"). Related: Quinarian.

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