Grammatically Correct: The writer's essential guide to punctuation ...

GRAMMATICALLY CORRECT
presented in Merriam Webster's Collegiate Dictionary. Another
source might differ on a few.)
Since the language is constantly evolving, not every compound
word will appear in the dictionary, and you will sometimes have to
make your own decisions. If the combination you want does not
have its own dictionary entry, you may usually assume it should be
written as two words. However, with compound words that are
relatively new to the language, the rules aren't hard-and-fast. There's
a general trend for a new compound to start out as open, to acquire
a hyphen as it becomes used more frequently and eventually to
merge into one word. Before a consensus is reached, all three forms
may be considered acceptable. For example, the computer terms
soft copy, soft-copy and sojtcopy all are commonly seen as both
noun and adjective.
COMPOUND NOUNS
With some compound nouns, more than one style may be acceptable.
For example:
carry-over or carryover
lay-offs or layoffs
make-up or makeup
send-off or sen doff
short-list or shortlist
Compound nouns that comprise more than two words, such as
idioms and phrases, usually take hyphens. This isn't invariable, however,
so always check-a good dictionary includes multiword phrases
and expressions. If a phrase does not have its own listing, assume
it does not take hyphens.
26
She's such a stick-in-the-mud.
He's a Johnny-come-lately.
The place was just a hole-in-the-wall.
But:
The deal sounded like a pig in a poke.
She was his partner in crime.
The beach was just a hop, skip and jump from the hotel.