IPA: /ˈhɛf.tɪ.li/
KK: /ˈhɛf.tɪ.li/
In a way that is heavy or substantial; with a lot of weight or force.
He heftily lifted the box onto the shelf without any help.
Heftily is derived from "heft" (from Old English "heft", meaning weight or heaviness) and the suffix "-ily" (meaning in a manner). The word "heftily" means in a heavy or weighty manner.
Think of 'heft' as relating to weight, and remember that 'heftily' describes doing something in a heavy or weighty way.