A phrase which conjures up spurious activities perpetrated by teenagers and other social malcontents which serve no function whatsoever beyond wasting the time of all involved.

Sometimes messing about can include destructive behavior but only so long as the damage is accidental. Intentionally wreaking havoc is a clear sign that the instigators have surpassed mere messing about; the single unifying theme in all such shenanigans is a disregard for anything that smacks of effort.

It also carries a strong British flavor, and is somehow more believable and forgivable when uttered by a native speaker of The King's English or at least a poseur with a passable accent.