There are always new words to learn even if you master a language rather well. My friend Emily sometimes teaches me dubious English idioms (“blowing the Popsicle stand” meaning to head out) and I teach her weird German sayings.
When I lived in Sweden, I carried a little book with me everywhere in which I jotted down what people would mention. The book is a hilarious collection of seemingly incoherent vocabulary. The more proficient you get in a language, the fewer opportunities to add words to the list. But I still keep a little list on my work desk and whenever my co-worker says something I don’t recognize, I pull it out and interrogate her: “What does this mean? When can I use it? Is this offensive or colloquial?” In my list so far, I have after three months, håglös (indolent), inte lägga fingrarna emellan (to make quick work of someone/something), jungfruben (split pins), räd (razzia) and sitta stilla i båten (to not rock the boat, interestingly, in German it becomes to keep your feet still). Now you can try to reconstruct our conversations – good luck!