When a girl is teasing a boy by mouthing "Olive Juice" to him, but with the perception she is mouthing "I Love You." Boy develops crush on girl, only to be found out later she actually had a crush on him as well. Slang.
"Teresa mouthed 'Olive Juice' to Tammy from across the room. Mike was in the middle and had a crush on Teresa. He thought she meant 'I Love You.' Years of confusion ensue."
Something you silently mouth to someone to return an "I love you". When you silently mouth "olive juice", it looks like "I love you". A silent "olive juice" is usually used when someone tells you they love you, and you want to keep them, but you're not ready for that.