Old English clafre, clæfre "clover," from Proto-Germanic *klaibron (cf. Old Saxon kle, Middle Low German klever, Middle Dutch claver, Dutch klaver, Old High German kleo, German Klee "clover"), of uncertain origin.
Klein and Liberman write that it is probably from West Germanic *klaiwaz- "sticky pap" (see clay), and Liberman adds, "The sticky juice of clover was the base of the most popular sort of honey." First reference in English to the suposed luck of a four-leaf clover is from c.1500. To be in clover "live luxuriously" is 1710, "clover being extremely delicious and fattening to cattle" [Johnson].
In a position of ease and affluence; happy as a clam (1710+)