(Strip Number 4719, Original Publication Date, 24 November 1986)
Panel 1: As the dog saunters by, Elly asks John is he's also noticed that Farley has been limping lately.
Panel 2: She goes on to say that whenever he runs or goes up the stairs, Farley seems to be in pain.
Panel 3: She wonders if they should get him checked because there might something wrong.
Panel 4: When Elizabeth suggests he may have "arfritis", John smiles behind his hand at the unintended pun.
Summary: This is harmless in and of itself. As an isolated strip, it's a kid lisping and a parent noticing the mildly amusing coincidence. As part of a wider pattern of John being a self-satisfied assbucket who says condescending bullshit about not intending to raise a princess because he's too stupid and arrogant to find out what's going on in his own damned house, it's NOT at all harmless.