During his conversation with Cecily and Miss Prism in Act 2, Part 1, Dr. Chasuble commits an unfortunate but meaningful slip of the tongue that involves a metaphor:
Chasuble: Were I fortunate enough to be Miss Prism's pupil, I would hang upon her lips. [Miss Prism glares.] I spoke metaphorically.—My metaphor was drawn from bees.
What Chasuble means to say, of course, is "hang upon her words," but his attraction to Miss Prism causes him to misspeak and sound much more suggestive than he intended. In an attempt to avoid embarrassment, he claims that he was actually making a reference to bearding, a behavior in which bees accumulate at the front of the hive in a beard-like shape. This metaphor, in addition to being ludicrously obscure, makes absolutely no sense in the context of the conversation.
Miss Prism, who is also secretly attracted to Chasuble, echoes his wording during a later conversation between the two:
Miss Prism: Maturity can always be depended on. Ripeness can be trusted. Young women are green. [Dr. Chasuble starts.] I spoke horticulturally. My metaphor was drawn from fruits.
Unlike Dr.Chasuble, Miss Prism purposefully uses figurative language when she compares young women to green or unripe fruit, with the implication being that she, as an older woman, is a "riper" romantic partner for Chasuble. When he balks at her forwardness, she attempts to undercut the sexual innuendo by calling attention to her use of metaphor, but this explanation does nothing to alter the meaning of her words.