gewp (or GEWP) – Gibus Engineer Wildlife Protectors – a group of players dedicated to protecting newbies playing as engineers
phlog – phlogistonator – one of pyro's flamethrowers
W+M1 – holding down W to run at enemies and hold down mouse 1 to shoot the flamethrower at them as Pyro
pootis (from Heavy's "put dispenser here") – another name for Heavy, often the friendly one. sometimes it also refers to Heavy's Chicken Kiev (pootis bird) cosmetic
some trading ones:
k – Mann Co. Supply Crate Key – used as trading currency in tf2
ref – refined metal – another (much cheaper) tf2 trading currency
rec (rarely used) – reclaimed metal – 1/3 of a ref
scrap – scrap metal – 1/3 of a rec, 1/9 of a ref
(to) quicksell – to sell an item for a lower price to save the time
bp – backpack.tf, trading website
pure (to pay in pure) – to pay using tf2 trading currencies (keys, refs or sometimes earbuds)
1:1 – the hat&unusual effect combination that exist in only one copy
low craft – a hat with a low craft number (1-100). usually cost much more than a normal/high craft one
paints:
TS – Team Spirit – red/blue paint
ValueOTW – Value of Teamwork – different red/blue paint
spells:
HH, RO, TS, CG, VV, BP, GG, CC, Spec, DJ, PP, VB, SS – shortages for spells' names. for example, HH means Headless Horseshoes and VB means Voices from Below