to make weak or feeble; to deprive of strength ...1647 obs.
from imbecility (n. mental or intellectual deficiency or weakness) + -ate
FIRST DOCUMENTED USE
1647 - see EXAMPLE below
"...What was the violent pursuing of Members but a stinking condition'd kinde of physicall fighting with the whole body by way of imbecillitating?..."
From: An Outcry Against the Speedy Hue & Cry after Generall Massey, Col, Poyntz and many other renowned reformadoes, etc.