IPA: /ˈhɛf.ti.nəs/
KK: /ˈhɛftɪnəs/
The quality of being heavy or having a lot of weight.
The heftiness of the package made it difficult to lift.
Heftiness is formed from "heft" (from Old English "heft", meaning weight or heaviness) and the suffix "-iness" (meaning the quality of). The word "heftiness" refers to the quality of being heavy or having weight.
Think of 'heft' meaning weight, and remember that 'heftiness' describes the quality of being heavy.