The story
From the fact that a word ending in /əɹ/ (usually ⟨-er⟩ or ⟨-or⟩) sounds like a verb followed by “her” as the direct object.
Etymology from Wiktionary, CC BY-SA 4.0.
I hardly know her
Today it means: A joking retort that draws attention to the possibility of sexual innuendo when a word ends with a schwar. Full definition →