Idiomatic expression is a word that has a meaning that is only known to a particular group of having a different meaning that is different from the usual meaning.
example of idioms;
break a leg - good luck
snake in the grass - a hidden enemy
closefisted - stingy or selfish
rainy day - a time of great need
busy as bee - fully occupied
eager beaver - a zealous worker
high spirit - cheerful mood
under the counter - secretly
lame duck - unfortunate
a barrel of monkey - very happy
going to the dogs- face critical situations