I'll say

I'll say

That's very true; I really agree; absolutely. A: "Wow, this bookshop has everything." B: "I'll say! I didn't even know copies of this book still existed!" A: "Did Billy enjoy his first day at school?" B: "I'll say he did! He didn't even want me to take him home at the end of the day."
See also: say
Farlex Dictionary of Idioms.

I'll say

Absolutely, I strongly agree. For example, Did you enjoy the film?-I'll say. This phrase is generally used alone and for emphasis. [First half of 1900s] For a synonym, see you can say that again.
See also: say
The American Heritage® Dictionary of Idioms by Christine Ammer.
See also:
  • (you) coulda fooled me
  • (you) could have fooled me
  • be wide of the mark
  • be/fall wide of the mark
  • at all
  • amaze
  • amazeballs
  • above and beyond the call of duty
  • bear a resemblance to
  • bear a resemblance to (someone or something)
References in classic literature
Harnish, who are you?" And I'll say, 'I'm Elam Harnish's younger brother.
"'Yes, ma'am,' I'll say, 'Burning Daylight was a pretty good cuss, but it's better that he's gone.
Burning Daylight
I'll say that he is wise who loveth well, And that the soul most free is that most bound In thraldom to the ancient tyrant Love.
Don Quixote
"I'll say a thank-you to him for that when I see him," said Anne, pulling easy chairs before the fire.
Annes House of Dreams
Give me the dog, and I'll say no more about the cat."
Michael Brother of Jerry
"I'll go to her; I'll ask her; I'll say for the last time: we are free, and hadn't we better stay so?
Anna Karenina
"Tom Sawyer," I says, "I'll say it again as I've said it a many a time before: I ain't fitten to black your boots.
Tom Sawyer Detective
I'll go a little further with you; I'll say something more."
Great Expectations
I assure you I'll say my prayers with a right good-will tonight.
Anne Of Green Gables
"I'll say it very quick," murmured the Other Professor, with downcast eyes, and melancholy voice, which contrasted oddly with his face, as he had forgotten to leave off smiling.
Sylvie and Bruno
"I'll say good-night to you here," she said, holding out her hand.
Of Human Bondage
"I'll say good-by by telegraph, when I get to London."
I Say No
If you an't the devil, Tom, you 's his twin brother, I'll say that for ye!"
Uncle Toms Cabin
You just say, 'Because she is,' and I'll say it's mere assertion.
Love of Life And Other Stories
But one thing I'll say, and no more; if you spare me, bygones are bygones, and when you fellows are in court for piracy, I'll save you all I can.
Treasure Island