In the UK, black person is the usual way to describe someone of African or Caribbean ethnic background and I wouldn't expect it to be taken as offensive. Referring to someone as a black (as a noun) would be offensive.
Referring to someone as the black guy could conceivably be interpreted as a little disrespectful if you might have been expected to call them by name, depending on the context. In your specific example you could have said I don't remember your colleague's name but he's black, if that helps? and I wouldn't expect anyone to be upset by that form of words.
Your friend is either misinformed or engaging in propaganda against perceived "political correctness". Stories about the word "black" being banned in some context or other pop up in the tabloid press with depressing regularity but invariably turn out to be untrue or misreported.
What is the meaning of the following line:
We ain't know what it meant
If you do a google search for 'we ain't know' you get quite a lot of returns for the phrase in a context in which didn't fits nicely.
These include:
1
We was young, we was dumb, we ain't know what was doing. But now that we older, older. Lets take these things slower, slower. Reset the button, button
2
And we ain't know just what he had in store. But now here we are ten years later, flows got greater. Bonds got closer, plus it's more haters
A lot of these uses of 'We ain't know' are in other rap songs. Rap has its own vocabulary.
In addition Wikipedia states:
some speakers of AAVE [African American Vernacular English] also use ain't instead of don't, doesn't, or didn't (e.g., I ain't know that).
Thus:
3
We ain't know it 'til General Wheeler come thru and tell us. After that, de massa and missus let all de slaves go 'cepting me; they kept me to work in de house and ...
That seems to clear the matter up, since didn't fits the context of the rap song "Yellow Brick Road" by (Eminem).
Here is the opening part of Verse 3 of the song:
My first year in 9th grade, can't forget that day at school
It was cool till your man MC Shan came through
And said that Puma's The Brand 'cause the clan makes troops
It was rumors but man, god damn, they flew
Must've been true because man we done banned they shoes
I had the new ones the Cool J, Ice land swayed too
And we just through them in the trash like they yesterday's news
Guess who came through next, X-clan debut
Professor X vanglorious exists in a state of red, black, and green
With a key sissies now with this being a new trend
We don't fit in crackers is out with Cactus albums
Blackness is in, African symbols and medallions
Represents black power and we ain't know what it meant
Me and my man Howard and Butter, we would go to the mall with 'em
All over our necks like we're showing 'em off not knowing at all
We was being laughed at you ain't even half black
You ain't supposed to have that homie let me grab that
And that Flavor Flave clock we gonna have to snatch that
Again, ain't fits perfectly as didn't.
The song is an auto-biographical narrative in which Eminem hearkens back to his first day(s) in 9th grade (a grade he failed twice before dropping out of high school) and remembering various events and moments. Not surprisingly an important topic is brands of athletic shoes, including Puma.
Eminem is a cracker, semi-offensive slang for a poor white boy. He attends a school with lots of black students. They make fun of Eminem when he and his friends wear African symbols and medallions. In the middle of this Eminem says we ain't know what it meant.
Ain't occurs 5 times in the song. All five instances are in Verse 3.
For those interested, here is youtube version that has the lyrics scroll as they are recited. There are a few mild-ish swear words. Stanza 3 begins right after the repeated chorus at 2:57.
Last, while I was pondering this question, I wrote the following section. It does not seem as relevant now, but I'm including it so that it wasn't a wasted effort. It also shows that you cannot always trust dictionaries. But then again, I've never been impressed with the ODO.
Ain't usually signifies
a negative form of to be
the negative form have not / has not
(Note: One of the ODO's examples for has / have not actually means aren't. This is the first example under 1.1 "More Example Sentences". In this example 'Baseball's origins ain't found till they're found', ain't means aren't. Not only is this clear from the clause itself; but the title is an allusion to the famous axiom of former New York Yankee catcher Yogi Berra: 'It ain't over till it's over.' In which ain't also signifies the verb to be. In English, axioms are stated in the present tense.)
Best Answer
Fun fact, etymologically, the word "to hum" is believed to have drawn its existence ultimately as the result of the sound being imitated.
This is more obvious when one uses the standard onomatopoeic transcription, which is "hmm". "Hmm" and "hum", I'm sure you can see the connection.
So yes, it absolutely can be described as a hum.