Meaning of "tough go"

I am trying to find out the meaning of "tough go" as in "This job is going to be a tough go for us."
While I can more or less guess it parhaps means a difficult activity, I would like to see a more dictionary type of definition. I tried to look it up on m-w.com but could not find it as a phrase. The various entries for go as a noun are not very helpful.
Actually, I always wonder if there are good online dictionaries where I can look up for phrases. If you have any suggestions, please point me to them.
Thanks to you all.