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Phrasal verb from light (“to make bright, ignite”, verb) +‎ up (“to a higher degree”, adverb).

light up (third-person singular simple present lights up, present participle lighting up, simple past and past participle lit up or (rare) lighted up)

1. (transitive) To illuminate, to bring light to something, to brighten.

   Synonyms: lighten; see also Thesaurus:illuminate

   • 1922 February, James Joyce, Ulysses, Paris: Shakespeare and Company, […], →OCLC:
     Episode 12, The Cyclops:

     The deafening claps of thunder and the dazzling flashes of lightning which lit up the ghastly scene testified that the artillery of heaven had lent its supernatural pomp to the already gruesome spectacle.

   • 2001, J. Stewart Burns, “Roswell That Ends Well”, in Futurama, season 3, episode 19, spoken by Philip J. Fry (Billy West):

     I've never seen a supernova blow up, but if it's anything like my old Chevy Nova, it'll light up the night sky.

   • 2020 April 22, “Network News: Glenfinnan turns blue to honour NHS workers”, in Rail, page 9:

     The illumination was organised by Martin Whyte from events company The Stage Group, fulfilling a long-held ambition of his to light up the 21-arch Scottish structure.

2. (intransitive) To start to emit light; to become activated as a light source.

   Synonyms: gleam; see also Thesaurus:shine

   • 2009, President Nixon's Pilot, Jim Bell, in The Propinquity Effect →ISBN:

     When we cranked up the engine, the fire warning light lit up.

3. (intransitive, figuratively) To show an increase in activity or a brightening of mood.
   • 1956 [1880], Johanna Spyri, Heidi, translation of original by Eileen Hall, page 84:

     Clara's eyes lit up at this highly unusual occurrence.

   He saw Mary and his face lit up.

4. (transitive) To ignite.

   Synonyms: inflame, kindle; see also Thesaurus:kindle

   • 1712 (date written), [Joseph] Addison, Cato, a Tragedy. […], London: […] J[acob] Tonson, […], published 1713, →OCLC, Act I, scene i, page 1:

     Absence might cure it, or a second mistress / Light up another flame, and put out this.

5. (intransitive) To light a cigarette, pipe, etc.

   Synonym: spark up

   Smoking in this building is not allowed, so I always step outside to light up.

   • 1952 February, J. Pelham Maitland, “Locomotive Working on Sussex Branches Fifty Years Ago”, in Railway Magazine, page 84:

     The cleaner worked, of course, at nights. He had to coal and light up the engine, as well as clean it, for the next day's work, which commenced with a light run to Barnham to "bring in the goods" from that station at about 6.30 a.m.

   • 1962 December, “Motive Power Miscellany: North Eastern Region”, in Modern Railways, pages 422, 425:

     On the morning after the one-day strike, October 4, one of the Type 4s on crew-training, No. D169, was appropriated to head the 3 a.m. mail to Hull, as no steam locomotive had been lit up and the usual Hull Type 3 was not available; [...].

   • 2019, Bernardine Evaristo, Girl, Woman, Other, Penguin Books (2020), page 15:

     where a group of women rolled up to enjoy the free plonk and spill out on to the pavement to light up and chat each other up

6. (transitive) To make happy.

   Synonyms: cheer up, happify, make someone's day; see also Thesaurus:gladden

   • 2001, Ash, Shining Light:

     You are a shining light, and you light up my life.

   • 2010, WLTX.com, Young Girl Continues Bike Giveaway Tradition, 25 Nov 2010

     "It lights me up, make me happy. Sometimes I go home, go in my room and cry with joy,"said Hudson smiling

7. (transitive) To introduce (someone to something), to make someone aware of or interested in something; turn on.
   • 1990 February 4, Pam Mitchell, quoting Gerry Scoppettuolo, “Pro-Union And Queer”, in Gay Community News, volume 17, number 29, page 17:

     Those clauses aren't all that uncommon these days; they were protecting thousands of gay and lesbian people before there were any city gay rights ordinances. That kind of stuff lit me up to the labor movement.

   • 2024 March 2, @Hannahsbenz17, Twitter‎^[1]:

     I never troll the other side but being in Florida for a few days & seeing the dude with the Gadsden “Don’t Tread on Me” shirt really lit me up to the reality of this nightmare we’re all trying desperately to survive & to keep our precious Democracy.

8. (transitive, slang) To open fire on a target or group of targets, especially with rockets, a flamethrower, etc.
   • 2007 September 25, Bungie, Halo 3, spoken by Marcus Stacker (Pete Stacker), Microsoft Game Studios, Xbox 360, level/area: The Ark:

     The back plating looks vulnerable, light that sucker up!

   telling me to light it up if it didn't stop approaching

9. (chiefly US, transitive, slang) To shock (someone) with a stun gun.

   Synonym: tase

10. (chiefly US, transitive, slang) To activate the emergency sirens on a police vehicle in order to pull someone over.

    I guess he doesn't realize who's behind him, because the way he's darting and weaving, he's practically begging me to light him up.

• lighten up

Phrasal verb from light (“to make less heavy”, verb) +‎ up (“to a higher degree”, adverb).

light up (third-person singular simple present lights up, present participle lighting up, simple past and past participle lighted up)

1. (transitive, nautical) To loosen, slacken, or ease off.

   to light up the jib-sheets

• William Dwight Whitney and Benjamin E[li] Smith, editors (1914), “light2”, in The Century Dictionary: An Encyclopedic Lexicon of the English Language, revised edition, volumes III (Hoop–O), New York, N.Y.: The Century Co., →OCLC, page 3447.

• uplight