The original meaning of premium is a reward given for some specific act or as an incentive; a prize. as per its etymology: Premium (n.) (Etymonline):
- c. 1600, "reward given for a specific act," from Latin praemium "reward, profit derived from booty,"
The OED cites early adjectival usage examples referring mainly to petrol especially in AmE:
(Passing into adj.) Of a commodity, etc., esp. petrol: superior in quality and therefore commanding a higher price; of a price: such as befits an article of superior quality; higher than usual. orig. U.S.
1928 National Petroleum News 24 Oct. 115 (Advt.), This is our anti-knock gasoline, a premium motor fuel.
While according to Etymonline its first usage was in 1925 referring to butter;
- Adjectival sense of "superior in quality" is first attested 1925, originally in reference to butter.
What's the origin of premium as an adjective? Was butter given as gift/prize, from which the sense of "top quality"? Was its origin from AmE usage because of its common reference to petrol or was "premium butter" a BrE expression?