Essentially just the words Squid (referring to the aquatic animal or cephalopoda) and Cum mashed together. Pronounced no different than the word "scum".
"Get that mouth open Patrick, I don't want you dropping any of this squm!"
"So anyway, I filled every orifice SpongeBob had with my squm."
Brought to public attention by Bob Woodward and Carl Bernstein in the book on the fall of Richard Nixon, All The President’s Men, ratfucker was Nixon’s term for a political operative who sows chaos among opponents; an agent provocateur. It is thought to have originated among campus political rivalries at USC, and seems to have some usage in the US military.
Along with Nixon’s other ratfuckers, Roger Stone caused fights at otherwise peaceful Democratic Party protests to generate a false narrative that the Democrats were violent and dangerous.