attagirl
An expression of encouragement, support, or approval toward a girl, woman, or female animal, used as a standalone interjection before or after a sentence. Largely presumed to be a corruption of the expression "that's the girl"; it is also sometimes spelled "atta girl." Attagirl! I knew you would ace your exam.
attaboy
An expression of encouragement, support, or approval toward a boy, man, or male animal, used as a standalone interjection before or after a sentence. Largely presumed to be a corruption of the expression "that's the boy"; it is also sometimes spelled "atta boy." You won the spelling bee? Attaboy!
attagal
An expression of encouragement, support, or approval toward a woman, used as a standalone interjection before or after a sentence. It is a variation of the more common (but also potentially diminutive) "attagirl;" sometimes spelled "atta gal." A: "I was promoted to executive manager this morning." B: "Attagal! You're in the big leagues now!"