1590s, probably a back-formation from grandiloquence. Related: Grandiloquently.
Entries linking to grandiloquent
grandiloquence n.
"lofty speaking or expression," 1580s, from Latin grandiloquentia, from grandiloquus "using lofty speech, bombastic," from grandis "big" (see grand (adj.)) + -loquus "speaking," from loqui "to speak" (from PIE root *tolkw- "to speak").