I've been riding a lot of bike since my early teens, when Pops suckered me into Bike Ride Across Georgia. I picked up the mountain bike in college, and have been riding the "dark side" ever since. Despite decades in the saddle, there's still a surprising amount of tribal jargon in the mountain biking community to learn. 

We're not gonna go full OED here, nor are we gonna even attempt to put down all the standard MTB terms and slang associated with mountain biking. Instead we will offer up a few of the more esoteric, whimsical, and ridiculous mountain biking terms, and make an attempt at a definition from this jargon. Here goes:

Berm
A banked turn made of dirt. Can be big, can be small. Best to crouch down and lean with the angle of the berm
"The best berms in North Carolina are at Warrior Creek. They're fast and giant."

Brap
This one is just too weird to really nail down a single definition. This comes close: http://www.leelikesbikes.com/definition-of-braaap.html

Crank
To pedal hard or those metal bars your pedals are stuck to
"We're gonna have to crank up to Ridgeline, but it'll be worth it for the 2 mile descent."

Double
See gap jump. I think British people say this.
"Oy, let's go hit those doubles then have a spot of tea with a pickles and cheese sandwich." (use a British accent)

Drop
A section of trail where it drops out from underneath you, ideally letting you catch like 3 feet of air. 3 inches of air is also acceptable. Any amount of air is fun. #noshametinyjumps
"Watch out after that first turn, there's a big rock drop that'll sneak up on you."

Gap Jump
A large jump with an entrance and a landing area, but a big gap in the middle. Terrifying most of the time, unless you know how to do it. Search MTB fails on YouTube and see why they're not to be messed with unless you've got the chops
"I will probably never hit big gap jumps because I have a healthy mortgage and 3 chilrden who need money for college."

Greasy
Slippery, such as a slippery trail made wet by rain, made greasy by water still on roots and rocks, which then slides like ice
"Those first few runs down Beech Mountain after last night's rain were super greasy."

Grom
A young child riding bikes, snowboarding, surfing, or doing outdoorsy things. Many times better than you.
"Those little groms are better riders than you."

Huck
Throw, chuck, carry, transport, but mostly means to hit a large jump with flat landing afterwards. This is basically the meaning of “huck to flat”. This one gets thrown around so much nobody really knows exactly what it means. Huckwagon, Hucking into the weekend.
"I can't believe Jeff hucked it off that sketchy wooden ramp. It was totally huck to flat. Huck yeah." 

Line
The path that you steer your bike on, attempting to avoid obstacles.
"Dude, that line you just took was sick".It was hard to pick a good line because the trail was full of greasy roots"

Manual
A wheelie
"Learning how to manual is really hard and takes way more practice than you think it will."

OTB
Over the bars. When your body takes the unnatural and unnerving flight over the handle bars because you hit something big, slammed on your front brakes, or descended something to steep. 
"Once as a kid I stuck a bar in spokes while riding my bike, and learned physics quickly, as I immediately went OTB." True story.

PBJ
Pumps, berms, jumps. Usually a specific section of a trail system that combines all three for a mini bike park vibe.
"The PBJ section at Rocky Knob is freaking fun."

Pump
The act of bouncing up and down on your bike, timed correctly with berms and rollers. By using your legs as giant shocks, you can pump up and down and pick up speed.
"You'll have an easier time going downhill if you pump over those rollers."

Pump Track
A self-contained and usually circular bike trail that has tons of fun little rollers and berms. If you do it right, which means pumping, you don't have to pedal.
"Somebody around here needs to open up a kid friendly beer garden with a pump track." Amen.

Roller
Fun little hills to roll your bike over. If you pump them right, you can accelerate.
"There are lots of fun rollers at Brumley Forest."

Send It / Full Send
What does “send it” mean in mountain biking? Hitting a jump of any size. Doing something amazing. 
"Yo girl, see that jump? Send it!" "That mountain bike festival was "full send"."

Schralp
When you speed through a tight turn or berm, applying downward pressure or brakes to your rear tire to make it spit up dirt like you’re a badass. Does damage the trail, but looks gnasty.
”Petty totally schralped those berms at Warrior Creek.”

Sick
Excellent. Enjoyable.
"Whooooaaaa! Sick…"

Sketch / Sketchy
Dangerous. Greasy lines are especially sketch.
"You gonna ride today? Naw, trails will be too greasy."

Steez
Doing something stylishly and with ease. Smash the two words together, and you get steez. Also - excellent.
"Todd sure does ride down Farlowe Gap with steez, despite how insanely rocky it is."

Step Down
See "drop"

Table Top
A big jump that's flat on top, which makes you less likely to hurt yourself, as opposed to a gap jump or double.
"San Lee has a bunch of nice table tops to practice your tail whips on."

Taco
Bending one of your wheels into an asymetrical shape because you hit something too hard. 
"I hit that tree stump, went OTB, and tacoed my wheel. It was sick."

Tail Whip
For amateus: Moving your bike sideways while in the air, best shown off when hitting a gap jump. Stylish and maybe functional, and takes lots of practice. For the pros: whipping the rear of the bike around in a full 360 in mid-air while still holding onto the handlebars.
"Watch me hit this gap jump and throw a giant tail whip down."

Of course, we're open to making a few additions here. Comments from the peanut gallery welcome.