go weak in the knees
go weak in the knees
To become unsteady or flustered due to having a strong emotional reaction to someone or something. Hearing the news of my father's sudden death, I went weak in the knees and could barely remain standing. Just thinking about Samantha makes me go weak in the knees.
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.