• (America) IPA: /ˈsɝʔn̩li/
• (RP) IPA: /ˈsɜːtn̩li/

certainly

1. In a way which is certain; with certainty.
   • 1624, Democritus Junior [pseudonym; Robert Burton], The Anatomy of Melancholy: […], 2nd edition, Oxford, Oxfordshire: Printed by John Lichfield and James Short, for Henry Cripps, OCLC 54573970 ↗:

     , I.iii.2.2:

     he verily thought he had young live frogs in his belly, qui vivebant ex alimento suo, that lived by his nourishment, and was so certainly persuaded of it, that for many years following he could not be rectified in his conceit.

2. Without doubt, surely.

   The accident was certainly caused by human error.

   That was certainly sweet of him.

   You may certainly join us for dinner.

   • 1908, W[illiam] B[lair] M[orton] Ferguson, chapter IV, in Zollenstein, New York, N.Y.: D. Appleton & Company, OCLC 731476803 ↗:

     So this was my future home, I thought! Certainly it made a brave picture. I had seen similar ones fired-in on many a Heidelberg stein. Backed by towering hills, […] a sky of palest Gobelin flecked with fat, fleecy little clouds, it in truth looked a dear little city; the city of one's dreams.

3. An emphatic affirmative answer; of course.

   Would you like it with ice?  Certainly, and with lemon please.

• (with certainty) absolutely, beyond doubt, indubitably, sure thing, undoubtedly, wis (obsolete), without a doubt
• (without doubt) definitely, doubtlessly, in fact, indeed, indisputably, indubitably, no doubt, really, sure, surely, truly, undoubtedly, unquestionably, wis (obsolete), without a doubt
• (emphatic affirmative) damn right, damn straight, fo shizzle, for sure, oh yeah, wye aye (dialect)

• French: certainement
• Italian: certamente
• Portuguese: certamente, garantidamente

• French: certainement, sûrement, sans nul doute, sans aucun doute
• German: sicherlich, zweifellos, gewiss, freilich
• Italian: certamente, senza dubbio, non ci piove
• Portuguese: certamente, sem dúvida
• Russian: коне́чно
• Spanish: ciertamente, sin duda, a todas luces

• French: certainement, bien sûr, pour sûr, évidemment
• German: sicher, natürlich
• Italian: certamente, evidentemente
• Portuguese: certamente, sem dúvida
• Russian: коне́чно
• Spanish: por supuesto