Tibeten~Sanskrit Idiom Query

[Stephen Hodge]

Dear Tibetanist Colleagues,

I have the expression "nyan-du mi-btub/ma-btub" which occurs about a dozen 
or so times in the Kanjur, particularly in the 'Dul-ba section.  Has anybody 
come across or can suggest a reliable / attested Sanskrit origin for this ~ 
contextually it seems to mean "disregard / ignore" something said.  I have 
looked in all the obvious sources, but cannot find anything.

Many thanks,
Stephen Hodge