You must be a scout, and you must have replaced your secondary weapon with a can of Bonk! Atomic punch. Bonk makes you briefly invulnerable.
Bonk is received for obtaining 22 scout achievements, as a random drop, or one possible result of crafting a scout class token+ secondary token + one scrap metal. It is also available on the community market. I currently see vintage bonk at 13 cents USA, or strange Bonk at 15 cents USA.
The best way to take 1000 points of damage is from a fully upgraded sentry turret. Find one, take cover, drink Bonk. Step out and get shot. You will suffer knockback and bonk will not Last long enough to get to 1000 damage. Retreat, and wait for your can of Bonk to recharge. Then, drink and step out again. Repeat in one life until the achievement is yours.