Verb. To investigate so as to discover the truth about a person or thing. Until Wes Davis suggested this to me, I had no idea it was a Britishism. It is. (Wes says he first heard it on BBC programs that aired here in the 1970s.) It seems to have originated in the sort of British noir slang sus, short for “suspect” (both the noun and verb). The OED cites D. Webb’s 1953 novel Crime Is My Business: “He turned to Hodge and said, ‘Who’s sussed for this job?'” This form first shows up in a 1966 article in Queen: “Youth susses things out on its own.” Google Ngram indicates that the expression turned up in the U.S. circa 1990 and has been steadily increasing in popularity ever since.
Fifty-nine to 36, 125 to 119, 5 to 2, 4 to 0; scores are all so obvious and pure — too damned obvious and pure for those of us inclined to suss out subtle meanings and unseen truths. (Kurt Andersen, Time, January 17, 1994)/We’ve taken a close look at all the pilots in production, talked to insiders, and read the scripts to suss out the twenty most exciting and intriguing ones, and then handicapped their chances of success. (New York Magazine, April 7, 2011)