False friends are words that sound similar enough between the language you know and the language you are learning that you think you know what they mean.
Many times this IS the case. A word sounds similar in English and it means the same thing in Portuguese. Such as:
fine=fina (at least in the thin, or finely ground flour sense of the word, pronounced FEE-na)
clip=clipe (like a paper clip, pronounced CLI-pee)
But it's the false friends that jazz up a conversation because they can be taken the wrong way, which may create tension or comedy. Or both. A few that I've encountered so far:
Emprego - this sounded like pregnant to me, and that's how I took it when someone told me that a woman they knew was emprego. I smiled and made a gesture of a big round belly and said "Ahhhh... bebê??!!!" To which they laughed and indicated that's not what they meant. A few days later I saw the word again in the want-ads of the newspaper. Emprego = job, or employed.
A viola in English is the big sister to the violin. In Portuguese a violão is a guitar.
Balconista - this is another word that I saw in the want-ads of the paper. Maybe it's just me, but my mind went to a pretty lady hanging off a balcony, possibly with some pole dancing involved?? My trusty dictionary informed me that a balconista is a sales clerk.
The biggest example between Spanish and Portuguese that I've heard so far is mas. Mas means more in Spanish, and but in Portuguese.
Off the topic of false friends, but on still on the topic of linguistics, I've noticed there is a one letter difference between luxury (luxo, pronounced LOO-sho), and garbage (lixo, pronounced LEE-sho). This screams of an excellent title for a trash-to-treasure television program on a home&garden type channel! Lixo ao Luxo - it must exist!!