Dic: begin to (for emphasizing the impossibility)

• You can't begin to imagine how much that saddens me.
• he couldn't begin to compete with her.
• I can't begin to imagine how awful it must have been.
• Those measures do not even begin to address the problem.

* * *

△ If you say that you cannot ^begin to^ imagine, understand, or explain sth, you are emphasizing that it is almost impossible to explain, understand, or imagine.

△ (used with a negative) to have the least capacity (to do sth)

△ (can't begin to understand/imagine etc) spoken. used to emphasize how difficult something is to understand etc

△ to do accomplish in the least degree.

.... COBUILD, COLLINS, LDOCE, American Heritage