Why did Beaver say, "There's trim there"?

Question

In the first half of the movie, Beaver says to Pete that he met a girl at Bingo and made love to her:

Pete: Bingo? Did you say bingo, like the game in church basements?

Beaver: There's trim there.

Pete: Beaver!

Answer 1

In this instance, he's using the term trim in its slightly less crude manner, to denote the fact that there are (sexually) available women there:

noun. A woman, esp one regarded as an object of sexual conquest (Circa 1955+)

Dictionary Central also has this to offer

noun: A female, especially an attractive girl. A briefly fashionable phrase used in hip circles and among medical students, etc. in the early 1990s. It was probably adapted from the (somewhat dated) black American use of the word to denote the female genitals...

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For the record, using this term in mixed company is likely to earn you a slap.

Answer 2

Trim is rude slang for female genitalia. The phrase in this context means that it was legitimate reasoning to meet a woman at bingo as there are women physically there.