oxalic acid means: Although oxalic acid can be found in many plants naturally, it is toxic in large quantities. There are measurable levels of oxalic acids in spinach, rhubarb and sorrel. Insoluble compounds of calcium and iron are formed by it. This reduces the nutritional value of spinach, rhubarb, and sorrel. (in Cooking Dictionary)

What else does oxalic acid mean?

A poisonous, colorless crystalline dicarboxylic acid, C 2 H 2 O 4 This is a natural bleach that can be found in many plants such as spinach and it’s used to remove rust and stain. (in Merlin Dictionary)