Etymology
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third degree (n.)

"intense interrogation by police," 1900, probably a reference to Third Degree of master mason in Freemasonry (1772), the conferring of which included an interrogation ceremony. Third degree as a measure of severity of burns (most severe) is attested from 1866, from French (1832); in American English, as a definition of the seriousness of a particular type of crime (the least serious type) it is recorded from 1865.

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Third World (n.)

1963, from French tiers monde, formulated 1952 by French economic historian Alfred Sauvy (1898-1990) on model of the third estate (French tiers état) of Revolutionary France; his first world (The West) and second world (the Soviet bloc) never caught on.

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black eye (n.)

"discoloration around the eye from injury" c. 1600, from black (adj.) + eye (n.). The figurative sense of "injury to pride, rebuff" is by 1744; that of "bad reputation" is from 1880s.

In reference to dark eyes, often as a mark of beauty, from 1660s. Black-eyed is from 1590s in reference to women, from 1728 in reference to peas. The black-eyed Susan as a flower name (various species) is by 1881, for their appearance. It also was the title of a poem by John Gay (1685-1732), which led to a popular mid-19c. British stage play of the same name.

All in the Downs the fleet was moored,
  The streamers waving in the wind,
When black-eyed Susan came aboard,
  "Oh! where shall I my true love find?
Tell me, ye jovial sailors, tell me true,
If my sweet William sails among the crew?"
[etc.]
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stink eye (n.)

"dirty look," by 1972, perhaps from Hawaiian slang.

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tertium quid (n.)

something indeterminate between two other things, 1724, Latin, literally "third something," from tertius "third, a third," from the root of tres "three" (see three). A loan-translation of Greek triton ti (Plato), used in alchemy for "unidentified element present in a combination of two known ones." The Latin word also figures in phrases tertium non datur "no third possibility exists," and tertius gaudens "a third party that benefits from conflict between the other two."

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terza rima (n.)

1819, Italian, literally "third rhyme." Dante's measure: a-b-a-b-c-b-c-d-c-, etc.

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lex talionis (n.)

1590s, legal Latin, "law of retaliation," an eye for an eye, a tooth for a tooth, from lex "law" (see legal) + talionis, genitive of talio "exaction of payment in kind" (see retaliation). Not related to talon. Other legal Latin phrases include lex domicilii "the law of the place where the person resides," lex fori "law of the place in which an action is brought."

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trompe l'oeil 

1889, French, literally "deceives the eye," from tromper "to deceive," a verb of uncertain origin and the subject of many theories (see trump (v.2)).

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morituri te salutant 

Latin, literally "those about to die salute you," words addressed to emperor by gladiators upon entering the arena. Third person singular is moriturus te salutat, first person singular is moriturus te saluto.

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sui generis 

1787, Latin, literally "of one's own kind, peculiar." First element from sui, genitive of suus "his, her, its, one's," from Old Latin sovos, from PIE root *swe-, pronoun of the third person (see idiom).

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