fool around

Example: “The boys were fooling around in the back of the classroom.”

Origin: The particle carries most of the meaning. English uses "around" to mark activity that has no direction or purpose — messing around, milling around, lounging around, hanging around, playing around — and in every case the effect is to strip the verb of any destination. To fool, as a verb meaning to jest or behave foolishly, is attested from the 1500s on its own; adding "around" specifies that the foolishness is not going anywhere in particular. That is why the phrase covers such different ground depending on context, from harmless idling to something considerably more pointed: the twentieth-century American sense of infidelity works precisely because it describes the conduct as aimless, which is the least flattering possible framing of it.