- cer|tain|lyW1S1 [ˈsə:tnli US ˈsə:r-] adv [sentence adverb]1.) without any doubt= ↑definitely▪ I certainly never expected to become a writer.▪ They're certainly not mine.it is certainly true/possible etc▪ It is certainly true that there are more courses on offer.▪ The girl was almost certainly murdered.▪ 'Not smoking has made a real difference.' 'It most certainly has.'2.) spoken used to agree or give your permission▪ 'I'd like a beer, please' 'Certainly, sir.'▪ 'Can I come along?' 'Certainly.'3.) certainly notspoken used to disagree completely or to refuse to give permission▪ 'May I go?' 'Certainly not!'
Dictionary of contemporary English. 2013.