What does Higgins mean by 'moment of extreme justifiable excitement'?

He is referring to a word beginning with "B" that the flower girl is using. Higgins believes that it is not refined to use such a word. It should only be used in a moment of "justifiable excitement".

This has to do with Higgins trying to remove inappropriate language from Eliza's vocabulary, all while using the inappropriate word (bloody) himself. It is not a word for polite society, but Eliza has always heard it, and Higgins uses the word himself when exasperrated or frustrated. Thus, when he's called on this bit of wordplay by Mrs. Pearce (he denies using the word); he admits that yes, he might have used it in a "moment of extreme justifiable excitement). Mrs Pearce notes how many of these moments he actually has....... while he's putting on his boots, or buttering his toast, or simply taking a piece of bread. By the time this moment of wordplay is done.... Mrs. Pearce has made her point.

Just in case you need to cite the text evidence, my copy begins this bit of wordplay between Mrs. Pearce and Higgins on line 703.

Thank you....!!!