1. To set something down clumsily or loudly. In this usage, a noun or pronoun can be used between "clunk" and "down." The guys lugged the couch inside and clunked it down in the middle of the living room.
2. To fall heavily or loudly. Did you hear that tree branch clunk down in the middle of the night?
clunk something down
to drop or place something heavily on something so that it makes a clunking noise. He clunked the big box down on the table. He clunked down all his school books on the table.
[for something] to drop or fall, making the sound "clunk." A large piece of metal clunked down right in front of me. A tree branch clunked down on the roof and damaged a few shingles.