Heavily means to a great degree, or with much weight. Rain heavily, heavily armed, sat down heavily.
It attaches to a small set of verbs and participles — heavily armed, heavily pregnant, heavily involved — and in most of them it simply means very. Sitting down heavily is one of the few uses about actual weight. Note heavy plus -ly.
1adverbheavilyTo a great degree, or with much weight.
“It rained heavily all night, and by morning the river had flooded.”
heavily is a degree.
Old English hefiglice is heavy plus the -lice ending that makes adverbs.
Proto-Indo-European
*keh₂p-
Old English
hefiġlīċe — heavily; grievously
Middle English
hevely
English, today
heavily
From Middle English hevely, hevyliche, from Old English hefiġlīċe (“heavily; grievously”), equivalent to heavy + -ly.
Etymology from Wiktionary, CC BY-SA