Go to Google and search for the word idiom plus the key words from your idiom. The search results are almost guaranteed to lead you to a helpful definition. Here is an example:
Imagine you heard this conversation:
Michael: Would you mind if I tried your new guitar?
Lori: Sure, knock yourself out!
The idiom is “knock yourself out,” or “to knock oneself out,” in the dictionary form.
To find the meaning of this idiom, do a Google search for the word
idiom
and add the phrase “knock yourself out” in quotation marks.
Like this:
idiom “knock yourself out”
All you need to do is scroll through the results, which, with luck, will be a combination of online dictionary definitions and online forum discussions. Make sure to check out any forum discussions that you find; they often contain interesting and useful discussions about meaning and usage and give you far more insight than you will get from a dictionary definition.
If you don’t get useful results on your first search, try adjusting your search query. For example, for “knock yourself out” you can try
idiom “knock oneself out”
or
idiom knock oneself out
Google is not a perfect corpus by a long shot, but it is a fantastic tool for doing quick’n'dirty research into meaning and usage. I highly recommend that you spend a few minutes reading the Google search help file so that you can take full advantage of this wonderful tool.
I use the trick I’ve shown you today all the time; in fact, Google is my number one reference tool for double-checking my native-speaker intuition. This is just one of several tips that I’ll be sharing with you over the coming weeks. I hope you find it useful.