To "
finger the bass" means to
finger a
vagina with skill. Bass guitarists are known to have skilled fingers, and fast hands, and often "finger" the notes to produce the sounds on their instrument. This is also a pun on the word "bass" and "bass", "bass" being short for a bass guitar, and "bass" being a type of fish. Vaginas are often referred to as being "fishy". Haha, great pun, I know!