avoir les jambes en marshmallow

to have legs that suddenly feel wobbly

Marshmallow is Quebec's spin on a much older French idiom family for legs that have gone soft and unreliable, usually from exhaustion, illness, or too much physical effort.

Après le marathon, j'avais les jambes en marshmallow pendant deux jours.

After the marathon, my legs were weak and shaky for two days.

This belongs to a family of French idioms that use a soft material for shaky legs, the classic version being 'jambes en coton', already used in Stendhal's 1839 novel La Chartreuse de Parme. Quebec's marshmallow is just the latest soft substitute.

Quebec