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English definition of “serious”

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serious

adjective uk   /ˈsɪə.ri.əs/ us    /ˈsɪr.i-/

serious adjective (BAD)

B1 severe in effect; bad: a serious illness There were no reports of serious injuries. The new tax regulations have landed some of the smaller companies in serious trouble. Drugs have become a serious problem in a lot of schools. This is a very serious offence. He's been taken to hospital where his condition is described as serious but stable. [after noun] mainly Indian English very ill
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serious adjective (NOT JOKING)

B1 not joking or intended to be funny: Please don't laugh - I'm being serious. He was wearing a very serious expression and I knew something was wrong. On the surface it's a very funny novel but it does have a more serious underlying theme.B1 A serious person is quiet, thinks carefully about things, and does not laugh a lot: I remember her as a very serious child.
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serious adjective (DETERMINED)

[after verb] determined to follow a particular plan of action: Is she serious about going to live abroad? [after verb] If two people who have a loving relationship are serious about each other, they intend to stay with each other for a long time and possibly marry: She's had a lot of boyfriends but Simon's the only one she's been serious about.
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serious adjective (NEEDING ATTENTION)

B2 [before noun] needing or deserving your complete attention: That's an interesting job offer - I'd give it some serious consideration if I were you. We've got some serious talking to do, you and me.
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serious adjective (EXTREME)

informal extreme in degree or amount: We did some fairly serious walking over the weekend. I mean we're talking serious (= a large amount of) money, right? informal very good of its type: This is a serious wine, Belle, you've just got to try some.
(Definition of serious from the Cambridge Advanced Learners Dictionary & Thesaurus © Cambridge University Press)
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