Antonym: perhaps. Similar words: certainly not, certain, uncertain, for certain, uncertainty, entertainment, mainly, ungainly. Meaning: ['sɜːtnlɪ] 
adv. definitely or positively (`sure' is sometimes used informally for `surely').
adv. definitely or positively (`sure' is sometimes used informally for `surely'). 
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181. The engine certainly needs tuning but there's nothing wrong with the car.
182. Smoking was certainly one of the factors that foreshortened his life.
183. They're certainly giving the book the hard sell, with advertisements every night on TV.
184. I'd had a couple of glasses of wine but I certainly wasn't drunk.
185. They may not be millionaires but they're certainly very comfortable.
186. The high tax policy is certainly unpopular, although it's unlikely to meet the soaring armament expenditures.
187. This is certainly a theoretical risk but in practice there is seldom a problem.
188. Jim never drank, and certainly didn't go a bundle on gambling.
189. She certainly has her hands full with four kids in the house.
190. We don't know each other that well. I certainly wouldn't bare my soul to her.
191. Our plan certainly paid off; it was a great idea.
192. News of the scandal certainly hastened his departure from office. Sentencedict.com
193. With profits at record levels, the company's certainly sitting pretty this year.
194. The timely rain after a long time of drought will certainly bring on the crops.
195. Consumers have certainly gained from the increased competition in the telecommunications industry.
More similar words: certainly not, certain, uncertain, for certain, uncertainty, entertainment, mainly, ungainly, curtain, maintain, concert, undertake, obtain, retain, sustain, captain, contain, container, retaining, sustainable, in line, only, openly, if only, only too, fall in love, in line with, commonly, only that, cancer.