US idiom? I try not to pick up my kids slang; makes me look desperate to fit in and silly. Never heard that one before; storing styptic pencils in a condom? Not my first choice.
I bought one last year when I started straight shaving, under advisement from @DaltonGang . I think I may have used it three times. At that rate of use; it's large enough to last my lifetime
My American slang is much better than it used to be. My wife on the other hand is almost totally lost. Her knowledge of English has improved quite a bit, though but she still has a way to go before she can reach my level. Except for my accent which refuses to go away I can speak English about 90% now fluently, as well as understand, read and write it about 98%. My wife speaks it now at around 70%, she can understand about 75% of what she hears as long as it's not slang, and she reads and writes it about 80%. Even I still get mixed up when I speak it occasionally!
I forgot to mention that unfortunately our daughter's English is perhaps 3% at speaking, 3% understanding, and 25% reading and writing. It's our fault for not pushing her to learn it. That's going to be changing a LOT when she starts school soon.
I was born and raised in central and western NY, so some idioms that are urban or military base just go right over my head. Many time I need to look them up online and the result is either being discussed or even more confusion.
A friend of mine here in Montreal is Greek. He lived in Israel for a while and his spoken Hebrew is close to perfect. He wasn't in the IDF but he knows the military slang very well!