#some douche made this up

Long ago, when something was yours and it started with a consonant (let’s say a boat), you called it ‘my boat’. When something started with a vowel and was yours (let’s say an elephant), you’d say… not ’my elephant’, but ’mine elephant’. This is fairly archaic language that only really exists in a few places anymore, but it’s very similar to the difference between 'a’ and 'an’.

The my/mine rule does have evidence scattered around history, though, particularly in naming patterns: You know the name 'Nancy’? It comes from 'Ann(e)’. Because parents would call their baby Anns 'mine Ann’, which became 'my Nan’, and then 'Nan’ and 'Nanny’ (the sense of 'nanny’ meaning aunt or caretaker comes from the Greek word for 'aunt’, 'nanna’, but the sense of 'nanny-goat'  comes from the nickname for 'Ann’) and eventually 'Nancy’. This process is similar to the evolution of the word 'nickname’.

Another name that got a similar treatment, though with much less permanence, was 'Ambrose’. Ambrose became 'Amb’, became 'mine Amb’, became 'mine Amby’, became 'my Namby’. Thus, the 'Namby’ in 'Namby-Pamby’ is referring specifically to someone named Ambrose.

But who? Well, a guy named Ambrose Phillip. He was a fairly well-known poet in the early 1700s, and also a politician. I can’t really tell you if he deserved the name 'namby-pamby’, I mean, I didn’t know him myself, but what I do know is that a man named Henry Carey didn’t like him much, and instead of going up to his house and trying to beat him senseless like Americans do, Carey wrote a poem about it:

“All ye Poets of the Age!
All ye Witlings of the Stage!
Learn your Jingles to reform!
Crop your Numbers and Conform:
Let your little Verses flow
Gently, Sweetly, Row by Row:
Let the Verse the Subject fit;
Little Subject, Little Wit.
Namby-Pamby is your Guide;
Albion’s Joy, Hibernia’s Pride.”

The poem proceeded to become very popular, and, because it was critiquing Phillips’ penchant for florid prose, the nickname became associated with everything sentimental and insipid, frivolous, weak or silly.

The 'pamby’ part of the equation, as far as I can tell, is fairly similar to rhyming things with words beginning with 'schm-’. So 'namby-pamby’ is kind of like saying 'Billy-Schmilly’, except with a lot more history.

Portamanteau is, like most words with too many letters, French. It first appeared in the 1540s, where it refereed to the guy who carried the mantle of the prince, from ‘porte’ (think 'porter’) and 'manteau’ (think 'mantle’). It took about fourty years for that word to mutate from the 'person’ part of a noun to the 'thing’ part, coming to refer to a type of suitcase.

And then, in 1882, some douchebag (you may have heard of him) decided it should have an entirely new meaning, and that meaning stuck. Since you can pack two things up at once in a suitcase– or a portamanteau– you can certainly pack two meanings up in one word.

And so a word completely changed its meaning due to some guy thinking it was a good idea at the time.