Definition of serious
1
: thoughtful or subdued in appearance or manner : sober
a quiet, serious girl
2a
: requiring much thought or work
serious study
b
: of or relating to a matter of importance
a serious play
3a
: not joking or trifling : being in earnest
a serious question
c
: deeply interested : devoted
a serious musician
4a
: not easily answered or solved
serious objections
b
: having important or dangerous possible consequences
a serious injury
5
: excessive or impressive in quality, quantity, extent, or degree
serious stereo equipment making serious money serious drinking
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Merriam-Webster unabridged
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Time Traveler for serious
The first known use of serious was in the 15th century
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