bad-hair day OR bad hair-day?

[Barnhart]

The more frequent of the two forms I have found is _bad hair day_.   I
have looked at the most recent 100 articles in Nexis.  The majority are
not helpful in determining the possible (bad (hair day) vs. ((bad hair)
day) problem.  Most are _bad hair day_.  About 20% are _bad-hair day_;
two (2) articles had _bad-hair-day_; none had _bad hair-day_.  This
interesting ambiguity is further intensified by the appearance of
_badhair day_ (in 8 of 15 examples) and _bad hairday_ (in 7 of 15
examples).   Needless to say the spell-checker didn't like _badhair_ or
_hairday_.

I guess it's not clear, at least from the Nexis evidence.

Regards,
David K. Barnhart, Editor
The Barnhart Dictionary Companion
barnhart at highlands.com