More than one right guess above but it is a tire checker(thumper.) I have one in the RV and I bought it at a truckstop. It takes a good bit of time to check all the pressures in 18 tires so you do that before starting out in the morning. What you fear is that one of a pair (dual) of tires will go flat, leaving the other to take the load. This will destroy both, Violently!You cannot see that it is flat and you will break a toe trying to kick one hard enough but it will sound dull when thumped so it is easy to check when stoping. It is also possible to put your palm on each and feel for the hot one. As has been mentioned above it is legal to have in the cab with you in case you need a weapon.