<a certain person told me that today is your birthday>
Origin of CERTAIN
Middle English, from Anglo-French, from Vulgar Latin *certanus, from Latin certus, from past participle of cernere to sift, discern, decide; akin to Greek krinein to separate, decide, judge, Old Irish criathar sieve
sure, certain, positive, cocksure mean having no doubt or uncertainty. sure usually stresses the subjective or intuitive feeling of assurance <felt sure that I had forgotten something>. certain may apply to a basing of a conclusion or conviction on definite grounds or indubitable evidence <police are certain about the cause of the fire>. positive intensifies sureness or certainty and may imply opinionated conviction or forceful expression of it <I'm positive that's the person I saw>. cocksure implies presumptuous or careless positiveness <you're always so cocksure about everything>.