Cleaner - an agent (or similar) whose job is to tie up loose ends and 'cleanse' the scene of hard evidence linking the agency or employer to the crime committed. May be a freelance, in which case it pays extremely well.
Dark - without official sanction, record or acknowledgement. Also termed 'off the books' or occasionally 'unsanctioned', although the latter indicates official disapproval.
Five by five - solid and reliable (intel), hale and hearty (agent).
Ghoul - A dangerous freelance operative, often one in competition with the speaker.
Rockstar - someone with an infinite appetite and capacity for drugs and/or alcohol.
Spook - Intelligence operative employed by a known/recognised agency.
Squint - (US origin) a derogatory term used by field agents and similar for the lab techs and support personnel. From their squinting into microscopes, and general myopia.
Stet (Medical origin) - Immediately and without question or discussion.
Wet(work) - Assassination and torture, both messy and illegal.
Black Ops - Unauthorised and officially non-existent branches of agencies, tasked with the killing and torture of targets deemed a threat to the 'parent agency' or country.
RED (Retired - Extremely Dangerous) - Former field agent or hired gun who has been forced out of active duty in some manner, but remains a very viable threat to any operative who might engage them.
Rogue - An agent or asset no longer under the control of the agency, but operating according to an agenda of their own, usually one which conflicts with the interests of the original agency.
Sanction - Permission, usually a kill authorisation. 'Extreme sanction' indicates a kill is desired, by any means necessary.
Under the radar - without attracting official attention from members of the speaker's own agency/side.
Some individual chars have personal usage beyond this, e.g. Joe Fixit/Idiolect.