what is the meaning of this sentence in an ironical atmosphere?

[Dary]

Hi,

what is the meaning of this sentence at answering of an ironical sentence?

I had to look that one up.

Thank you.

[Mister Micawber]

.
I didn't understand it-- I had to look it up in my dictionary.
.
_________________
Native English teacher at Mister Micawber's

[Dary]

Thank you so much Mister Micawber.