When learning a foreign language, a 'false friend' is a word in the language you are learning which is identical or similar to a word in your own language but has a completely different (and specifically, inappropriate or misleading) meaning.
I have managed to find a few examples from German (I had to search on the net...my 'O' level German was only good for two words). The word on the left is German, with its English translation after the hyphen.
Gift - poison
Gymnasium - secondary school
Puff - brothel
Rat - advice
Tripper - gonorrhoea
