retroflex
adjective
ret·ro·flex ˈre-trə-ˌfleks 
1
: turned or bent abruptly backward
2
: articulated with the tongue tip turned up or curled back just under the hard palate
retroflex vowel
Word History
Etymology
New Latin retroflexus, from Latin retro- + flexus, past participle of flectere to bend
First Known Use
1776, in the meaning defined at sense 1