Why were 18th century male trend-setters known as dandies or macaronis?

Answer 1

"dandy (n.)"
"c.1780, of uncertain origin, first appeared in a Scottish border ballad:"

"I've heard my granny crack
O' sixty twa years back
When there were sic a stock of Dandies O"

"etc. In that region, Dandy is dim. of Andrew. In vogue in London c.1813-19. His fem. counterpart was a dandizette (1821) with Fr.-type ending. The adjective dandy first recorded 1792; very popular c.1880-1900. "

"macaroni"

"1599, from southern It. dialect maccaroni (It. maccheroni), pl. of *maccarone, possibly from maccare "bruise, batter, crush," of unknown origin, or from late Gk. makaria "food made from barley." Used after c.1764 to mean "fop, dandy" (the "Yankee Doodle" reference) because it was an exotic dish at a time when certain young men who had traveled the continent were affecting Fr. and It. fashions and accents. There is said to have been a Macaroni Club in Britain, which was the immediate source of the term."

Answer 2

DANDY - Etymology: probably short for jack-a-dandy, from jack + a (of) + dandy (origin unknown)

MACARONIS - [Macaroni Club, a group of such Englishmen] a : a member of a class of traveled young Englishmen of the late 18th and early 19th centuries who affected foreign ways - I assume that 'macaroni' being of Italian origin (maccheroni, plural of maccherone, from Italian dialect maccarone dumpling, macaroni) this was a generic or slang name taken to those who affected foreign ways