attagirl
1 of 2interjection
at·ta·girl ¦a-tə-¦gərl 
—used to express encouragement, approval, or admiration to a woman or girl
They also wore ponytails out the backs of their hats and shouted "Attagirl!" every time a fielder made a nice stop in the dirt.
Brian MacPherson, Arizona Republic, 20 June 2006
Brian MacPherson, Arizona Republic, 20 June 2006attagirl
2 of 2noun
at·ta·girl ¦a-tə-¦gərl
plural attagirls
chiefly US, informal
: an expression of encouragement, approval, or admiration given to a woman or girl
I've had plenty of friends tell me they read these weekly updates and offer encouragement. I've also had lots of readers I don't know send attagirls or dieting suggestions by e-mail. Susan Pierce, Chattanooga (Tennessee) Times Free Press, 13 May 2010
Susan Pierce, Chattanooga (Tennessee) Times Free Press, 13 May 2010
Word History
Etymology
Interjection
alteration of that's the girl (after attaboy entry 1)
Noun
derivative of attagirl entry 1
First Known Use
Interjection
1917, in the meaning defined above
Noun
1974, in the meaning defined above