Apart from the main meaning, the word pismo is used on a daily basis as a soft cursing word (like blast!, damn! or darn!).
Svoji punci je poslal ljubezensko pismo.
(He sent a love letter to his girlfriend.)
Pismo je bilo naslovljeno na njegovega brata.
(The letter was addressed to his brother.)
Včasih je bilo dopisovanje prek pisem bolj pogosto.
(Conversation by means of letters used to be more frequent in the past.)
Pismo mi gre ta ženska na živce.
(Damn! I can’t stand that woman.)
Pismo! A se lahko umakneš?
(Damn! Can you move aside?)
Pismo as a curse word likely comes as a children-friendly substitute for a much juicier pizda (cunt). A comprehensive list of Slovenian curse words, ranging from soft to hardcore, can be found here. Yes, it is a rather long list, and most of them are encountered frequently in various situations. Good luck.