Glossary
Thrifty
In one line: Winning a round in which your team spent far less than the enemy.
What it means
Thrifty is a small banner the game shows when you win a round with much less equipment than the enemy: pistols against rifles, for example. It is the reward for a well-played eco.
For the economy, a thrifty hurts the team that suffers it: they spent a lot, lost their guns and handed the other team money and confidence. That is why the anti-eco is played carefully.
An example
You are on an eco with pistols and the defense has rifles. You stack up on a tight angle, win the close-range fights and plant. The round ends with "Thrifty": you won while spending almost nothing.