heavy: carrying great weight, and by extension anything large, severe, or hard to bear
At its root this word describes literal weight, something with real heft that presses down: a stone, a load, hands too tired to stay raised. From that plain physical starting point the Bible stretches the same image in every direction, and English has to keep changing the word to follow it.
A weight can be a quantity, so a herd or an army or a caravan is called heavy when it is large and imposing. A weight can be a hardship, so a famine, a plague, or a spell of mourning is heavy when it bears down on people with crushing force. A weight can be a duty, so a task or a governing burden or a labor-yoke is heavy when it is more than a person can carry. And a weight can dull a moving part, so a mouth, a tongue, or a heart is heavy when it will not respond, working slowly or not at all.
All of these are one picture seen from different sides. The reader who holds onto the sense of downward pressure will recognize why the same Hebrew word can mean weighty, severe, numerous, burdensome, and sluggish, and why translators reach for a different English word almost every time.