tongue-tied
adjective uk/ˈtʌŋ.taɪd/ us/ˈtʌŋ.taɪd/
（通常因紧张而）张口结舌的，说不出话的
If you get tongue-tied, you find it difficult to express yourself, usually because you are nervous.
Synonyms: inarticulateincoherent
Opposites: articulate fluent (LANGUAGE)