1 : to make thin or slender
2 : to make thin in consistency : rarefy
3 : to lessen the amount, force, magnitude, or value of : weaken
4 : to reduce the severity, virulence, or vitality of <an attenuated virus>
I came across it in The Gunslinger
It would have to be quick, or she would leave him. To stay now meant attenuation; perhaps her own kind of death. Already he felt her drawing away to leave the circle of the stones.