Online Google Dictionary

low-key wordnet sense
Adjective
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Not elaborate, showy, or intensive; modest or restrained,
  1. Not elaborate, showy, or intensive; modest or restrained
    • - their wedding was a very quiet, low-key affair
  2. Having a predominance of dark or muted tones

Verb
  1. Behave or speak with restraint
    • - a very simple, low-keyed style