Without a second thought
Definition 1 of 1
Pro Tip 1/3Immediate Action
Use it to describe a quick and decisive action taken without hesitating or thinking it over. 
She offered to help with the charity event without a second thought.
Pro Tip 2/3Emotional Impulse
Indicates a response driven by strong feelings or emotions, often used with acts of kindness or bravery. 
Moved by the plight of the homeless, he donated his winnings without a second thought.
Pro Tip 3/3Not Literal
The phrase is often not literal; there might be some thought, but it's essentially quick and decisive. 
When she saw the sale, she bought the dress without a second thought.