"aware, up-to-date," first recorded 1908 in "Saturday Evening Post," but said to be underworld slang, of unknown origin. Variously said to have been the name of "a fabulous detective who operated in Cincinnati" [Louis E. Jackson and C.R. Hellyer, "A Vocabulary of Criminal Slang," 1914] or a saloonkeeper in Chicago who "never quite understood what was going on ... (but) thought he did" [American Speech, XVI, 154/1]. Taken up by jazz musicians by 1915. With the rise of hip (adj.) by the 1950s, the use of hep ironically became a clue that the speaker was unaware and not up-to-date.
"about, at or near (a given date)" when the exact time is unknown, 1856, from Latin circa (adv., prep.) "around, round about, near; in the region of; about the time of," alternative form of circum "round about" (see circum-).