metallic element, 1812, Modern Latin, altered from earlier platina, from Spanish platina "platinum," diminutive of plata "silver," from Old French plate or Old Provençal plata "sheet of metal" (see plate (n.)). Related: Platiniferous.
The metal looks like silver, and the Spaniards at first thought it an inferior sort of silver, hence the name platina. It was first obtained from Spanish colonies in Mexico and Colombia, brought to Europe in 1735, and identified as an element 1741. Taken into English as platina (1750), it took its modern form (with element ending -ium) in 1812, at the time the names of elements were being regularized.
As a grayish-white color (similar to that of the metal) it is attested by 1923; especially as a shade of blond hair, it is attested by 1927 (in platinum blonde "woman with platinum-blonde hair;" Jean Harlow, famously associated with the label, starred in a popular movie of that name in 1931).
There is a blonde type to me more irresistibly lovely than all the rest of the women who come under the "preferred" classification. She is the platinum blonde. ... There are thousands of blondes with gold in their hair, and as many with red, and quite as many again with the yellow of corn, some real and sometimes corned with the aid of a well known bleaching ingredient—but the platinum blonde maintains her supremacy by her rarity. [Antoinette Donnelly, beauty advice column in New York "Daily News," Jan. 25, 1927]
As a designation for a recording that has sold at least one million copies, platinum is attested from 1960 ("The Battle of New Orleans"); in the late '50s it had been used to commemorate 3 million sales of a record (Pat Boone "Love Letters in the Sand"), and, from 1954, 25 million in total record sales (Jo Stafford, Gene Autry).
c. 1300, "determined, fixed," from Old French certain "reliable, sure, assured" (12c.), from Vulgar Latin *certanus, extended form of Latin certus "determined, resolved, fixed, settled," of things whose qualities are invariable, "established," also "placed beyond doubt, sure, true, proved; unerring, to be depended upon" (also source of Old French cert, Italian certo, Spanish cierto), originally a variant past participle of cernere "to distinguish, decide," literally "to sift, separate." This Latin verb comes from the PIE root *krei- "to sieve," thus "discriminate, distinguish," which is also the source of Greek krisis "turning point, judgment, result of a trial" (compare crisis).
Transferred sense, in reference to persons, "full of confidence in one's knowledge or judgment, made certain in reference to a matter or thing," from mid-14c. (also a sense in Latin). Meaning "established as true beyond doubt" in English is from c. 1400. Meaning "indefinite, not specifically named, known but not described" is from late 14c.
Different as this seems to be from sense I, it is hardly separable from it in a large number of examples: thus, in [a certain hour], the hour was quite 'certain' or 'fixed', but it is not communicated to the reader; to him it remains, so far as his knowledge is concerned, quite indefinite; it may have been, as far as he knows, at any hour; though, as a fact, it was at a particular hour. [OED]
Lewis & Short write that Latin certus also was sometimes indefinite, "of things, the certainty of whose existence is given, but whose nature is not more definitely designated, or comes not into consideration ...."
Hence the euphemistic use, attested from mid-18c., as in woman of a certain age "an old maid;" woman of a certain description "disreputable woman;" in a certain condition "pregnant;" a certain disease "venereal disease;" of a certain weight "obese." Used with proper names from 1785, "often conveying a slight shade of disdain" [OED]. Certainer, certainest were common to c. 1750, but have fallen from proper use for some reason. Expression for certain "assuredly" is attested by early 14c.