The adjective stilted describes something—usually a style of writing or speaking—that is unnaturally formal.

Imagine someone walking awkwardly on a pair of stilts and you have a good impression of the word stilted — wooden and stiff. Stilt is found in the mid-15th century, referring to walking on wooden stilts across marshy ground. A hundred years later, the word stilted came to refer to the posts holding up a building. It wasn't until 1820 that the word was used as we use it now, to describe writing or speaking that does not flow smoothly.

Definitions of stilted

adj artificially formal

“a stilted letter of acknowledgment”
“when people try to correct their speech they develop a stilted pronunciation”
artificial, contrived, hokey
affected, unnatural
speaking or behaving in an artificial way to make an impression