A variation of the mushroom stamp
(slap), where one's penis is covered with semen and/or residue left over from a good anal sex session. The icky is the semen and/or residue, the thump is the sound of the stamping (slapping) penis.
Sally was a little agitated after I gave her the ol' icky thump, but after another go at things and a shower she piped down.
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