late 14c., grant "large, big" (early 12c. in surnames), from Anglo-French graunt and directly from Old French grant, grand (10c., Modern French grand) "large, tall; grown-up; great, powerful, important; strict, severe; extensive; numerous," from Latin grandis "big, great; full, abundant," also "full-grown;" figuratively "strong, powerful, weighty, severe," of unknown origin.
In Vulgar Latin it supplanted magnus and continued in the Romanic languages. The connotations of "noble, sublime, lofty, dignified," etc., were in Latin. In English it developed a special sense of "imposing." Meaning "principal, chief, most important" (especially in titles) is from 1560s; that of "of very high or noble quality" is from 1712. As a general term of admiration, "magnificent, splendid," from 1816. Related: Grander; grandest.
Grand jury is late 15c. Grand piano from 1797. The grand tour of the principal sites of continental Europe, as part of a gentleman's education, is attested by that name from 1660s. The Grand Canyon of the Colorado River in western U.S. was so called by 1869, popularized by Maj. John Wesley Powell, scientific adventurer, who explored it; earlier it had been known as Big Canyon. For grand slam see slam (n.2).
-ly 2
common adverbial suffix, forming from adjectives adverbs signifying "in a manner denoted by" the adjective, Middle English, from Old English -lice, from Proto-Germanic *-liko- (cognates: Old Frisian -like, Old Saxon -liko, Dutch -lijk, Old High German -licho, German -lich, Old Norse -liga, Gothic -leiko); see -ly (1). Cognate with lich, and identical with like (adj.).
Weekley notes as "curious" that Germanic uses a word essentially meaning "body" for the adverbial formation, while Romanic uses one meaning "mind" (as in French constamment from Latin constanti mente). The modern English form emerged in late Middle English, probably from influence of Old Norse -liga.