turn to jelly

turn to jelly and feel/be like jelly{of legs or knees} (to suddenly feel weak because you are frightened, nervous or ill) — стать ватными, подкоситься (в выраж. ноги стали ватными, ноги подкосились) ; затрястись, задрожать (в выраж. коленки задрожали, поджилки затряслись )

If your legs or knees feel like jelly, they start to shake because you are frightened or nervous (LDOCE)

Example 1: With legs slowly turning to jelly, I skied down the steep slope. (MM)
Example 2: She couldn’t move – her head was swimming, her mouth was dry and her legs felt like jelly.
Example 3: As she knocked on the director's door, her legs turned to jelly. (CALD)

Related vocabulary:
get the jitters /the willies
in [butterflies in one's stomach]
[blood runs cold]
[chicken out]
[get cold feet]
[yellow-bellied]
[мурашки]
[поджилки трясутся]
[наложить в штаны]

[кровь стынет в жилах]
[дрожать как осиновый лист ]
[мороз по коже]