I recommend watching TV shows like "Big Bang Theory", "How I Met Your Mother," or "Friends." Pay attention to Sheldon's character in "Big Bang Theory" because the other characters need to describe sayings and phrases to him since he takes everything literally. Sometimes reading comics like "Peanuts" or "Dilbert" can help. Native speakers will use them without much thought so the idioms can be hard to pick up in conversation. In case you didn't know already, someone "under [the] weather" means that someone is sick.

The expression is "under the weather". There are many idiomatic expressions discussed on You Tube and various internet sites. I would just type in esl idiomatic expressions and you will find many. The best way to learn them is to find them used in context and then try to associate them with something similar in your own language. If nothing exists, you need to memorize them .Remember that all idiomatic expressions that you find are commonly used. Try to pay attention to what you hear used in movies , songs and tv shows. Some used in reading might not still be in use or not commonly used. Ask when you hear them used, if you don't understand them.

"How can I learn idiomatic expressions?" You need to learn them precisely. For example, "I feel under weather" is not correct. In fact, it makes you sound very awkward. We only say, "I feel under the weather". A half-correct idiom either changes the meaning or loses its meaning entirely.

Thank you AL. You are so nice!

I forgot to mention, this set of children's books are useful for learning idiomatic expressionshttp://en.m.wikipedia.org/wiki/Amelia_Bedelia