Molo

A Xhosa greeting used to greet one person. To greet multiple people, you say molweni.
Some people spell it as mholo.

"Molo sisi, kunjani? Uphilile na? Bendicel'umzuzu wokuthetha nawe mna" - Mandisi Dyantyis

Marete

A baSotho/Tswana term for Balls (As in testicles, not a soccer ball).

O boa marete saan

Matric

The last year of high school (grade 12).

I can't wait to do my matric.

Matric Dance

Also known as Matric Farewell - Is when grade 12 (last year of high school) learners have one final formal function, often followed by parties, underage drinking and bad decisions. Each school usually has its own dance and pupils usually come with a date and wear expensive suits and dresses.
Americans would call it prom

I don't know who I am going to take to the Matric Dance.

Mahala

A Zulu term that means for nothing. Free; obtained without payment or difficulty. It is however not only used by Zulu, it has been incorporated to modern South African lingo.

Buy 1 drink and get the second one mahala.

Mntkbw

Xhosa term derived from Mntakabawo which directly translates to "My dad's child". This term doesn't necessarily refer to someone you are related to. It can be used to refer to a friend or any random person.

I'm so tired mntkbw.

Moer

To beat someone to a pulp.

Can also be a swear word in Afrikaans

I will moer you.

Mdidi

A vulgar or explicit way to say butt or ass.

Do not say this to anyone or you will be moered. 

You are mad, mdidi wakho. 

Mjolo

Mjolo basically means dating. It's derived from ukujola which means to date. Therefore mjolo is the act of dating (ukujola).

Other people may spell it as mojolo

Mjolo will nyisa you. 

Mr 082

A guy who trended for what was apparently a staged video. He was asked for his number by a woman in a car and he kept saying "082". Twitter gave him the name Mr 082. 

Did you see the Mr 082 video?