To kill, to eliminate physically. Literally ”to wet”, derives from Russian cant in which a ”wet deal” way a business involving blood spilling.
Если, пардон, в туалете их [террористов] поймаем, и в сортире замочим.
(Путин В.В., 1999 г.)
If we, pardon me, find them [terrorist] in the toilets, we will polish them off in the john.
(V. Putin, 1999)