A slang term for a woman's genitalia, specifically when aroused. This is seen in Tom Robbins Still Life With Woodpecker (Bantam, 1980) as Princess Leigh-Cheri refers to her own bits as "the half-shellfish half-peach that occupied the warm, watery bowl of her nether regions" on page 20.
"She'd descovered that silk had a tendency to agitate the peachfish." (22-23)
"A sudden jolt shot through her peachfish, fuzz and fin." (73)