Altiloquent - High-flown or pretentious language.
Gilderoy Lockhart hadn't always used such vibrant, flowing language. In fact, as a child, he'd been a late talker and limited himself to the shortest possible sentences because of how shy he was..
It was only when he was about twelve, just after he'd started Hogwarts that he realised how much fancy language could help.
It was his first visit home and he'd copied something one of his Professors had said. His mother stared at him in astonishment.
"I didn't know you knew that word, Gilderoy!"
He basked in her pride and promised himself he'd do anything to maintain it.