Literally it means "I do not know" in French. But it is actually an euphemism to express a pleasant or desirable trait about something or someone that can't be desribed or explained.
This French idiom is frowned upon by people who feel socially, economically, or mentally threatened because they can't stand "snobs" who use this phrase (see
first definition in this list). People who fear foreign languages also have a tendency to have small reproductive organs, carry big guys to compensate for their lack of confidence, and are generally
socially inept when trying to carry on an
intelligent conversation with people smarter then them.
"There's a certain lack of
je ne sais quoi in people who call other people snobs..."
"I don't know about that guy over there, there's definitely no je ne sais quoi in his life."
"That poor
unintelligent bastard over there doesn't like people who say je ne sais quoi."