As well as being a type of bag, a portmanteau is also a word made up of two other words, like ‘smog’ (smoke and fog) or ‘breathalyser’ (breath and analyser). It was Lewis Carroll who first used ‘portmanteau’ in this way. Here’s Humpty Dumpty in ‘Through the Looking Glass’:
‘Well, “slithy” means “lithe and slimy” ... You see it’s like a portmanteau – there are two meanings packed up into one word.’
Modern slang continues to embrace the portmanteau with words like ‘chillax’, ‘bromance’ and, a personal favourite, ‘craptacular’ (something that’s so crap it’s also spectacular).
Weirdly, and rather pleasingly (at least if you’re a wordy geek like me – maybe not so much if you’re a normal person), ‘portmanteau’ itself is a portmanteau. It's made up of ‘porter’, French for ‘to carry’ and ‘manteau’, also French for cloak. MIND. BLOWN.