Oscilloscope reading human body
If you have an unearthed plug or perhaps a miss wired plug or socket you may find that the scope is internally weakly earthed (via a drain resistor or coupling cap) to what they think will be the neutral wire in the mains input.  This will cause the scope to have a earth reference to be floating at some fraction of the mains voltage. 

The peak to peak voltage of 115V RMS is about 162V p-p.
Alternately you may find that the scope is close to the correct ground reference but your body is at close to main potential due to capacitive coupling or resistive leakage to some other mains apparatus in the vicinity.  The moderately high input impedance of the scope will allow any stray signals that different from the scope ground reference to be accurately displayed.

You could test by grounding the scope (they often have grounding points on the dash and grounding yourself and will likely see the signal disappear.  Alternately you can usually enhance the signal by touching the metal case of plugged in equipment, a florescent tube in a fitting or even holding a insulated mains cable and insulating your self from earthed objects.

When doing measurements it is advisable to minimise such stray signals before trying to trouble shoot circuits even though correct connection of the earth clip will usually remove the artefact in practice, sometimes it is nice to be able to probe with no earth connected