Here are a few that came to mind,these aren't my terms, just some I've heard before or stumbled across. A few might be a stretch, some perhaps don't qualify as slang:
Postholed- breaking through the top layer of snow into the mushy stuff below, typically up above the knees and into the crotch.
Hiker trash- referring to PCT hikers specifically?
UL- ultralight, it's more of an acronym, but it get's thrown out there in a casual manner on this forum and in conversation pretty frequently.
bagged- kind of a climbers term, but we use it as well, many of us backpackers "bag" peaks on trips.
tanking- loading up on water between sources, usually during long stretches without available water.
MYOG- make your own gear, it's another acronym of course, but the manner in which it's used is bordering slang usage.
GORP- depending on who you talk to and what's in it, is a hiker term used when referring to trail mix, "good ol raisins and peanuts" is one version.
bushwhack- hike off trail
blaze- term used when referring to a trail indicator found on a tree, blazes vary.
wetted out- usually when a materials DWR properties fail, or a materials surface area has been fully saturated
DAM- down air mattress
singletrack, double track- pretty self explanatory
ditty bag- the small stuff sac of personal odds and ends that make their way into a backpack. Ex: chapstick, iPod, matches, keys, toothbrush and paste, contact solution, batteries, etc.
trimmed- removing excess material and ounces from an item of gear for the sake of lightening the load.
meths- short for methylated spirits, usually referring to various liquid fuels (denatured alcohol, HEET, acetone) used in lightweight stoves.
vitamin I- ibuprofen, not very specific to hiking, but some hikers use ibuprofen for pain management
mid- generic term when referring to a pyramidal style shelter pitched with one pole or 2 combined, ex: Oware, Mountain Laurel Designs Duomid, Megamid, and Speedmid, Z-packs Hexamid, Black Diamonds Betamid, etc.
LNT- leave no trace
potty trowel- tool used for digging cat holes to drop a deuce in.
Ti- all things titanium
"Westie"- a product manufactured by the reputable and highly adored company Western Mountaineering, specifically their sleeping bags
skin out- term used by super ultralighters and ultralighters alike to refer to the inclusion of everything that is worn from the "skin out" in your gear list, so anything on you and in your pack goes on your gear list and is added to the total weight.
blow out- epic failure of a portion of your trail running shoes (shoes, upper, toe guard), usually resulting in unsightly duct tap repairs and haggard sewing jobs in the field.
fastpacking- verdict is still out on what this term even means now. Traveling very light, and covering lots of mileage very fast...?
bomber- slang term referring to an item of gear that is extremely durable
nest- referring to a lightweight hammock users setup: (hammock, suspension, underquilt, overquilt, tarp, etc.)
crotch rot- the cumulative effect of spent days in the backcountry neglecting hygiene in the downstairs region of the body.
monkey butt- general nastiness in the region of the buttocks, brought on by sweaty pants, poor wiping, using the Mike C. method of toilet paperless wiping, chafe, etc.
Maybe that's a start.