From decisive +‎ -ly.

• (UK) IPA^(key): /dɪˈsaɪsɪvli/

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decisively (comparative more decisively, superlative most decisively)

1. In a decisive manner.
   • 1891, Thomas Hardy, Tess of the d'Urbervilles, volume 1, London: James R. Osgood, McIlvaine and Co., page 6:

     'In short,' concluded the parson, decisively smacking his leg with his switch, 'there's hardly such another family in England.'

   • 2011 November 5, Phil Dawkes, “QPR 2 - 3 Man City”, in BBC Sport‎^[1]:

     The visitors rallied again though and struck decisively 16 minutes from time when Yaya Toure headed in the winner.