GR translation: Frisking his great mustache

[Mike Jing]

V479.3-10   The forenoon has gone dark. Rain spits in under the awning. The
bowl of porridge has stayed after all untouched in front of Morituri.
Slothrop is sweating, staring at the bright remains of an orange. “Listen,”
it has occurred to his agile brain, “what about Bianca, then? Is she going
to be safe with that Greta, do you think?”
       Frisking his great mustache, “What do you mean? Are you asking, ‘Can
she be saved?’ ”
       “Oh, pip, pip, old Jap, come off it—”

What action does "frisking one's mustache" actually entail?  It already
appeared a couple of times in the book, and I just found I still don't
really understand it.
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