Certain means sure beyond doubt. I am certain I locked it. It is certain to rain. It is the strong end of a scale that runs through likely and possible.
But it has a second, quite different job, and the two live side by side: a certain means a particular one, not named. A certain person, a certain amount, certain conditions. That use is vague on purpose, which is the opposite of the first sense being definite, and learners often meet the vague one without realising it is the same word. Note that certain of and certain about both work for the sure sense, and that the noun is certainty, which behaves as you would expect.
1adjectivecertainCompletely sure.~11%
“She was certain she had posted it.”
2adjectivehappenSure to happen.~2%
“Defeat looked certain by half-time.”
3adjectiveparticularA particular one, not named.~85%
“A certain member of this family keeps eating the biscuits.”
4adjectiveadjectiveSome, but not much.~2%
“There is a certain amount of truth in that.”
Shares are an estimate: a language model read 18 sentences found in the wild and placed 18 of them on a sense — a count of our sample, not a measurement of English.
certain is a degree of confidence.
Latin certus meant settled or decided, the past participle of cernere, to sift or distinguish — the same verb behind discern and concern. Something certain has been sifted out from what is doubtful.
Proto-Indo-European
*krey-
Latin
certus — fixed, resolved, certain
Old French
certain
Middle English
certeyn
English, today
certain
Built from
From Middle English certeyn, certein, certain, borrowed from Old French certain, from a Vulgar Latin - unattested form certānus, extended form of Latin certus (“fixed, resolved, certain”), of the same origin as cretus, past participle of cernere (“to separate, perceive, decide”). Displaced native Middle English - wis, iwis (“certain, sure”) (from Old English ġewiss (“certain, sure”)) and alternative Middle English - spelling sertane (“some, certain”).
Etymology from Wiktionary, CC BY-SA