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the act of riding a female doggy-style while whipping pelvis in a circular motion much like a lasso in an attempt to catch the vagina with testicles. If one or both balls penetrate or are engulfed by the vagina a montana lasso has been performed.
Bob: While hitting it doggy-style I started dipping my hip in a circular movement so my balls would swing up and I could montana lasso Alexis.

Chris: Did you montana lasso her?