Long answer: "lying with a man" with the implication of "as you would with a woman"
Short answer: gay sex
Is the Tanakh's word for bed (משכב) ever used in modern Hebrew? Is it perhaps used as a euphemism for sex? If so, is it commonly used?
Sometimes you use it for a "high languege" (I have no idea how you say that in Inglish, in hebrew its "שפה גבוהה". hope you understand..
It's sometimes used in fixed expressions, like "משכב זכר", but even they aren't that common.