At | Mean of at in English Dictionary

/ˈæt/

• because of or in reaction to (something)
1. They laughed at my joke.
2. She's furious at how slowly the investigation is proceeding.
3. I was greatly surprised at [=by] the result. [=the result surprised me greatly]
4. He enlisted in the army at his father's urging. [=because his father urged him to]
5. She visited the museum at my suggestion.
6. They came here at our invitation.
7. You can act at your own discretion.
• doing some activity
1. I didn't expect her to have started working yet, but she was already (hard) at it when I arrived. [=she was already working (hard) when I arrived]
2. My neighbors are always arguing, and they were at it again last night. [=they were arguing again last night]
3. Since we're cleaning the kitchen, we should wash the floor while we're at it. [=we should wash the floor when we clean the kitchen]
4. “I'm going to the store to buy a newspaper.” “While you're at it, could you get some milk?”