1. Given to laughter
2. Inclined to incessant or foolish merriment
Comes from Abdera, a town in Thrace of which place Democritus, the Laughing Philosopher was a native.
Example: I'm usually the serious type of person more inclined to point out why an absurd commercial is improbable than to find it funny, but luckily that unyielding sensibility has also driven me to surround myself with people able to turn me into a helplessly abderian fool whose main concern is to keep milk from spraying out her nose at the next quip.