Green’s Dictionary of Slang

sudden; unexpected.

1860		Surtees Plain or Ringlets? (1926) 22: [S]uccessive pop visits by the fair ladies to Mrs Baccoman’s looking-glass.
1866		Bradford Obs. 6 Dec. 6/6: I paid a pop visit to a very elegant mansion at Holy croft.
1896		Grantham Jrnl 4 Jan. 8/6: A Pop Visit to the Blankney.